docs: Add page describing SeaBIOS final object linking

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

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+This page describes the process that the SeaBIOS build uses to link
+the compiled code into the final binary objects.
+
+Unfortunately, the SeaBIOS linking phase is complex. This complexity
+is due to several unusual requirements:
+
+* Some BIOS entry points must reside at specific hardcoded memory
+  locations. The build must support positioning code and variables at
+  specific locations.
+* In order to support multiple [memory models](Memory Model) the same
+  C code can be complied in three modes (16bit mode, 32bit segmented
+  mode, and 32bit "flat" mode). Binary code from these three modes
+  must be able to co-exist and on occasion reference each other.
+* There is a finite amount of memory available to the BIOS. The build
+  will attempt to weed out unused code and variables from the final
+  binary. It also supports self-relocation of one-time initialization
+  code.
+
+Code layout
+===========
+
+To support the unusual build requirements, several
+[gcc](http://en.wikipedia.org/wiki/GNU_Compiler_Collection) compiler
+options are used. The "-ffunction-sections" and "-fdata-sections"
+flags instruct the compiler to place each variable and function into
+its own
+[ELF](http://en.wikipedia.org/wiki/Executable_and_Linkable_Format)
+section.
+
+The C code is compiled three times into three separate objects for
+each of the major supported [memory models](Memory Model):
+**code16.o**, **code32seg.o**, and **code32flat.o**. Information on
+the sections and symbols of these three objects are extracted (using
+**objdump**) and passed in to the **scripts/layoutrom.py** python
+script. This script analyzes this information and produces gnu
+[ld](http://en.wikipedia.org/wiki/GNU_linker) "linker scripts" which
+provide precise location information to the linker. These linker
+scripts are then used during the link phase which produces a **rom.o**
+object containing all the code.
+
+Fixed location entry points
+---------------------------
+
+The build supports placing code entry points and variables at fixed
+memory locations. This support is required in order to support the
+legacy BIOS standards. For example, a program might execute an "int
+0x15" to request system information from the BIOS, but another old
+program might use "ljmpw $0xf000, $0xf859" instead. Both must provide
+the same results and so the build must position the 0x15 interrupt
+entry point in physical memory at 0xff859.
+
+This support is accomplished by placing the given code/variables into
+ELF sections that have a name containing the substring
+".fixedaddr.0x1234" (where 0x1234 is the desired address). For
+variables in C code this is accomplished by marking the variables with
+the VARFSEGFIXED(0x1234) macro. For assembler entry points the ORG
+macro is used (see **romlayout.S**).
+
+During the build, the **layoutrom.py** script will detect sections
+that contain the ".fixedaddr." substring and will arrange for the
+final linker scripts to specify the desired address for the given
+section.
+
+Due to the sparse nature of these fixed address sections, the
+layoutrom.py script will also arrange to pack in other unrelated 16bit
+code into the free space between fixed address sections (see
+layoutrom.py:fitSections()). This maximizes the space available and
+reduces the overall size of the final binary.
+
+C code in three modes
+---------------------
+
+SeaBIOS must support multiple [memory models](Memory Model). This is
+accomplished by compiling the C code three separate times into three
+separate objects.
+
+The C code within a mode must not accidentally call a C function in
+another mode, but multiple modes must all access the same single copy
+of global variables. Further, it is occasionally necessary for the C
+code in one mode to obtain the address of C code in another mode.
+
+In order to use the same global variables between all modes, the
+layoutrom.py script will detect references to global variables and
+emit specific symbol definitions for those global variables in the
+linker scripts so that all references use the same physical memory
+address (see layoutrom.py:outXRefs()).
+
+To ensure C code does not accidentally call C code compiled in a
+different mode, the build will ensure the symbols for C code in each
+mode are isolated from each other during the linking stage. To support
+those situations where an address of a C function in another mode is
+required the build supports symbols with a special "\_cfuncX_"
+prefix. The layoutrom.py script detects these references and will emit
+a corresponding symbol definitions in the linker script that points to
+the C code of the specified mode. This is typically seen with code
+like:
+
+`extern void _cfunc32flat_process_op(void);`\
+`return call32(_cfunc32flat_process_op, 0, 0);`
+
+In the above example, when the build finds the symbol
+"\_cfunc32flat_process_op" it will emit that symbol with the physical
+address of the 32bit "flat" version of the process_op() C function.
+
+Build garbage collection
+------------------------
+
+To reduce the overall size of the final SeaBIOS binary the build
+supports automatically weeding out of unused code and variables. This
+is done with two separate processes: when supported the gcc
+"-fwhole-program" compilation flag is used, and the layoutrom.py
+script checks for unreferenced ELF sections. The layoutrom.py script
+builds the final linker scripts with only referenced ELF sections, and
+thus unreferenced sections are weeded out from the final objects.
+
+When writing C code, it is necessary to mark C functions with the
+VISIBLE16, VISIBLE32SEG, or VISIBLE32FLAT macros if the functions are
+ever referenced from assembler code. These macros ensure the
+corresponding C function is emitted by the C compiler when compiling
+for the given memory mode. These macros, however, do not affect the
+layoutrom.py reference check, so even a function decorated with one of
+the above macros can be weeded out from the final object if it is
+never referenced.
+
+Code relocation
+---------------
+
+To further reduce the runtime memory size of the BIOS, the build
+supports runtime self-relocation. Normally SeaBIOS is loaded into
+memory in the memory region at 0xC0000-0x100000. This is convenient
+for initial binary deployment, but the space competes with memory
+requirements for Option ROMs, BIOS tables, and runtime storage. By
+default, SeaBIOS will self-relocate its one-time initialization code
+to free up space in this region.
+
+To support this feature, the build attempts to automatically detect
+which C code is exclusively initialization phase code (see
+layoutrom.py:checkRuntime()). It does this by finding all functions
+decorated with the VISIBLE32INIT macro and all functions only
+reachable via functions with that macro. These "init only" functions
+are then grouped together and their location and size is stored in the
+binary for the runtime code to relocate (see post.c:reloc_preinit()).
+
+The build also locates all cross section code references along with
+all absolute memory addresses in the "init only" code. These addresses
+need to be modified with the new run-time address in order for the
+code to successfully run at a new address. The build finds the
+location of the addresses (see layoutrom.py:getRelocs()) and stores
+the information in the final binary.
+
+Final binary checks
+===================
+
+At the conclusion of the main linking stage, the code is contained in
+the file **rom.o**. This object file contains all of the assembler
+code, variables, and the C code from all three memory model modes.
+
+At this point the **scripts/checkrom.py** script is run to perform
+final checks on the code. The script performs some sanity checks, it
+may update some tables in the binary, and it reports some size
+information.
+
+After the checkrom.py script is run the final user visible binary is
+produced. The name of the final binary is either **bios.bin**,
+**Csm16.bin**, or **bios.bin.elf** depending on the SeaBIOS build
+requested.