add a loopback.cfg

As reported in FS#71916

I would like to see a loopback.cfg file on each ISO produced by archiso. A loopback.cfg is basically just a grub.cfg that's designed to be used to boot a live distribution from an iso file on a filesystem rather than an actual physical CD. This is a standard that has been adopted by several distributions by now.

https://www.supergrubdisk.org/wiki/Loopback.cfg

Here are examples of loopback.cfg https://github.com/jim945/autoiso-multiboot/blob/master/loopback/arch/loopback.cfg

And the config to run it https://github.com/jim945/autoiso-multiboot