mkarchiso: improve functions and comments
- Merge
_make_efi_dir_on_iso9660
into_make_bootmode_uefi-x64.systemd-boot.eltorito
since it's specific to that boot mode. - Split off FAT image creation into
_make_efibootimg
, so it can be reused by other functions. - Add/improve comments to better document what the script does.