Adds call to mknod if loop dev does not exist after call to losetup -f (#483)

export-noobs/prerun.sh

@@ -11,7 +11,7 @@ rm -rf "${NOOBS_DIR}"
 
 echo "Creating loop device..."
 cnt=0
-until LOOP_DEV="$(losetup --show --find --partscan "$IMG_FILE")"; do
+until ensure_next_loopdev && LOOP_DEV="$(losetup --show --find --partscan "$IMG_FILE")"; do
 	if [ $cnt -lt 5 ]; then
 		cnt=$((cnt + 1))
 		echo "Error in losetup.  Retrying..."