Unmount device on overlay failure before continuing to next device. Overlay support works from different device without unmounting the first device, so this might be a bug in BusyBox.

src/08_generate_rootfs.sh

@@ -101,6 +101,7 @@ for DEVICE in /dev/* ; do
       OUT=\$?
       if [ ! "\$OUT" = "0" ] ; then
         echo "  Mount failed (probably on vfat), moving on with other devices."
+        umount /tmp/mnt 2>/dev/null
         continue
       fi
     else