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.
7 jobs for bootmodes-2 in 38 minutes and 40 seconds (queued for 7 seconds)