Fix umount_image (#22)

* 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 close RPi-Distro/pi-gen#19

scripts/common

@@ -52,7 +52,7 @@ unmount_image(){
 	sleep 1
 	local LOOP_DEV=$(losetup -j ${1} | cut -f1 -d':')
 	if [ -n "${LOOP_DEV}" ]; then
-		local MOUNTED_DIR=$(mount | grep $(basename ${LOOP_DEV} | head -n 1 | cut -f 3 -d ' '))
+		local MOUNTED_DIR=$(mount | grep $(basename ${LOOP_DEV}) | head -n 1 | cut -f 3 -d ' ')
 		if [ -n "${MOUNTED_DIR}" ]; then
 			unmount $(dirname ${MOUNTED_DIR})
 		fi