Unify common pieces to "embedded" targets (those that produce code
that runs neither in the host nor in a com32 environment); this fixes
the broken sample/ directory Makefile.

Cleanup and centralize the Makefile system even more.
Fix a gcc 4.3 incompatibility in memdisk (definition of strlen).

Move source files out of the root directory; the root is a mess and
has become virtually unmaintainable.  The Syslinux core now lives in
core/; the Linux and generic utilities has moved into utils/, and
copybs.com has moved into dos/; it had to go somewhere, and it seemed
as good a place as any.

Use $(CC) in gcc_ok macro, not plain gcc.  This seems to work, iff the
gcc_ok macro is declared with =, not :=

Avoid using commas in filenames, since not all systems can handle
them.

Add a .bin suffix to generated filenames