[libc] Redefine low 8 bits of error code as "platform error code"
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>
Michael Brown