[efi] Add efifatbin utility

Add utility for constructing EFI fat binaries (dual 32/64-bit
binaries, usable only on Apple EFI systems).

This utility is not part of the standard build process.  To use it:

  make util/efifatbin bin-i386-efi/ipxe.efi bin-x86_64-efi/ipxe.efi

and then

  ./util/efifatbin bin-*-efi/ipxe.efi fat-ipxe.efi

Requested-by: Brandon Penglase <bpenglase-ipxe@spaceservices.net>
Tested-by: Brandon Penglase <bpenglase-ipxe@spaceservices.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>