Even if GRUB is not used as a boot loader for the ISO, create a
`/boot/grub/grubenv` file in the ISO 9660 file system. If a
`loopback.cfg` file exists in the profile's `grub` directory, copy it
to `/boot/grub/loopback.cfg` on the ISO.

This ensures the funtionality will not be lost if the used boot loaders
are changed.