Ensure loop device partition nodes are created (#741)

Although the loop block device is created before attaching the image
to it, the devices for the partition that the image contains are still
not created. This patch creates those devices as well, when they are
not already available.

Fixes #482

Signed-off-by: Vasilis Tsiligiannis <acinonyx@openwrt.gr>

export-noobs/prerun.sh

@@ -22,6 +22,7 @@ until ensure_next_loopdev && LOOP_DEV="$(losetup --show --find --partscan "$IMG_
 	fi
 done
 
+ensure_loopdev_partitions "$LOOP_DEV"
 BOOT_DEV="${LOOP_DEV}p1"
 ROOT_DEV="${LOOP_DEV}p2"