Move efiboot.img out of the ISO file system

As said in #48 (closed), the only use for the /EFI directory on the ISO file system is to allow copying files off it to a FAT formatted flash drive. The /EFI/archiso/efiboot.img file is useless for that use case.

Instead of placing it as a file in the ISO file system, why don't we attach it as a partition instead. It's more compliant with UEFI-2.4 according to xorrisofs(1).

efiboot.img would remain in ${work_dir}/ and be attached to the ISO. It can be mapped in MBR and El Torito using xorrisofs options:

-append_partition 2 0xef "${work_dir}/efiboot.img" \
-e --interval:appended_partition_2:all::

or in GPT and El Torito:

-append_partition 2 0xef "${work_dir}/efiboot.img" \
-appended_part_as_gpt \
-e --interval:appended_partition_2:all::

or in GPT and El Torito in another way:

-efi-boot-part "${work_dir}/efiboot.img" \
-e --interval:appended_partition_2:all::

According to xorrisofs(1), the only issue is with multi-session runs. If I understand it correctly, that means the attached partition will be lost if changes are made to the ISO file system on unfinalized discs. But we don't really support making changes to the ISO after it's been built.