* export-noobs/prerun.sh: Use nested mountpoint
While it seems elegant and intuitive to use separate bootfs and rootfs
mountpoints for compressing the partitions, doing so violates a
precondition of unmount_image that they be mounted as a tree. This
causes the image to not be properly unmounted and detached. A better
solution might be to pack up the previous stage's chroot directory, but
that rework can wait for the time being.
scripts/common.sh: Output device name correctly
A misplaced ) in unmount_image caused the loop device to be incorrectly
identified, resulting in a fair bit of chaos trying to unmount other
filesystems on /dev/mapper devices. Such as / on a LUKS-encrypted
installation, for example. The unmount will fail as it should and
build.sh will abort the build without any cleanup. Best to avoid that.
These changes closeRPi-Distro/pi-gen#19
