Handle filesystems loop-mounted on file images

Handle filesystems loop-mounted on file images

Improve prepare_grub_to_access_device to emit appropriate commands for
such filesystems, and ignore them in Linux grub.d scripts.

This is needed for Ubuntu's Wubi installation method.

This patch isn't inherently Debian/Ubuntu-specific.  losetup and
/proc/mounts are Linux-specific, though, so we might need to refine this
before sending it upstream.  The changes to the Linux grub.d scripts
might be better handled by integrating 10_lupin properly instead.

Patch-Name: mkconfig-loopback.patch

Gbp-Pq: Name mkconfig-loopback.patch