Add '-e' (or -eltorito-boot-efi) option for writing an EFI eltorito boot

Add '-e' (or -eltorito-boot-efi) option for writing an EFI eltorito boot
image, in addition to or instead of the x86 boot image, to the output file.
Per the UEFI standard, the EFI boot image should be an image of an EFI
System Partition to be loaded in no-emulation mode, so we always record it
that way rather than checking if it looks like a floppy image.

with, tested by and ok yasuoka@
strong encouragement from deraadt@