Move most arch/i386 files to arch/x86, and adjust the contents of the
Makefiles and the include/bits/*.h headers to reflect the new
locations.

This patch makes no substantive code changes, as can be seen using a
rename-aware diff (e.g. "git show -M5").

This patch does not make the pcbios platform functional for x86_64; it
merely allows it to compile without errors.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Relicense files for which I am the sole author (as identified by
util/relicense.pl).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

The low 8 bits of an iPXE error code are currently defined as the
closest equivalent PXE error code.  Generalise this scheme to
platforms other than PC-BIOS by extending this definition to "closest
equivalent platform error code".  This allows for the possibility of
returning meaningful errors via EFI APIs.

Signed-off-by: Michael Brown <mcb30@ipxe.org>