[efi] Fix minor typos in efi_image.c

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Add EFI-specific debugging macros

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[int13] Split out ISO9660 and El Torito definitions to separate header files

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