[archiso] mkarchiso: optionally add EFI "El Torito" boot support.

If EFI/archiso/efiboot.img exists then an alternative "El Torito"
boot image is added to the ISO image compatible for EFI.
This image is a FAT filesystem, that is interpreted by EFI as ESP
(EFI System Partition).

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>