When update-grub is run, it will detect the disks in the build system.
Currently, we sed the grub configuration in places where this happens;
replace_grub_root_with_label moves that seddery in to a single callable
location to avoid ending up with multiple versions of it.
(It's worth noting that we also do this in 999-cpc-fixes.chroot, but as our
functions file isn't available within the chroot we don't use it there.)
We skip the previous unmount/remount behavior because that makes things
unwieldly when there are multiple partitions; but we still have setup to do,
so that /dev is available for grub, etc.
- added ability to create derivative images
- added function to convert images to qcow2
- cleaned up the loop clean up logic
- added function to umount and settle devices
