Never generate empty backup name

* Backup would then go to /sdcard/TWRP/*serial_num*/
  (without any subfolder) and restore would not be able to find it.

Signed-off-by: Vojtech Bocek <vbocek@gmail.com>
Remove unneeded images
Merge "Remove unneeded images" into twrp2.7
Merge commit 'fd44bf2c05e2b51c82c3de22114fa127fdcba919' into twrp2.7