Now that we have multiple firmware backends it no longer makes sense
to write object files to the same directory as their source. A better
solution is to write the object files to a per-firmware directory
under a top-level object directory.

The top-level object directory can be specified on the command-line
with the O= variable, e.g. make O=/tmp/obj. If no top-level object
directory is specified an 'obj' directory is created in the top-level
of the Syslinux source repository.

All the existing make targets continue to work as before, however now
they apply to all firmware backends, e.g. 'make installer' will build
the BIOS, 32-bit EFI and 64-bit EFI installers and place them under
$(OBJ)/bios, $(OBJ)/efi32 and $(OBJ)/efi64 respectively.

Note unlike every other bit of Syslinux, the gpxe objects are still
kept in the src directory, e.g. gpxe/src, since gpxe is only required
by the BIOS backend.

It is possible to specify a make target for a specific firmware or
list of firmware with the following syntax,

    make [firmware[,firmware]] [target[,target]]

To clean the object directory for just the BIOS firmware type,

      'make bios clean'

To build both the 32-bit and 64-bit EFI installers type,

     'make efi32 efi64 installer'

Since the Syslinux make infrastructure is now more complex a new file
doc/building.txt has been created to explain how to build Syslinux.

The top-level Makefile now exports some make variables for use in
module Makefiles,

    - topdir - the top-level source directory of the Syslinux
      repository, e.g. /usr/src/syslinux

    - objdir - the top-level object directory for the firmware
      backend currently being built, e.g. /obj/syslinux/bios

    - SRC - the source directory in the Syslinux repository for the
      module currently being built,
      e.g. /usr/src/syslinux/com32/libupload

    - OBJ - the object directory for the module currently being
      built, e.g. /obj/syslinux/bios/com32/libupload

Since we're rewriting the Makefile infrastructure anyway it seemed
like a good idea to add parallel support. By writing subdirectories as
prequisites for make targets the objects in those subdirectories can
be built in parallel.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>

Add additional codepage data from the Unicode Consortium.
Additionally, add support for generating codepage files for the case
where the filesystem codepage and the display codepage do not match.
The standard Makefile doesn't actually do that, however.

Make the codepage data all derive from the Unicode tables, and create
files that could be dynamically loaded in the future.

Initial support for VFAT long filenames; currently hard-coded to
codepage 865, no support for other codepages or for accessing files
with names which aren't in the current codepage.  This hopefully
shouldn't be an issue for SYSLINUX applications.

Add support for creating codepage/Unicode mapping tables; this is a
step toward VFAT long filename support.