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 branch 'twrp2.7' of git://github.com/TeamWin/Team-Win-Recovery-Project
Fix custom theme support