lime-sysupgrade: add BACKUP_EXTRA_DIR functionality
lime-sysupgrade: add BACKUP_EXTRA_DIR functionality

Current backup functionality needs that the files backed up are present
in the current firmware. This new option allows storing new files into
/tmp/lime-sysupgrade/preserve/ that will be saved to the new firmware.

libremesh/lime-packages,libremesh/lime-packages,libremesh/lime-packages,libremesh/lime-packages,libremesh/lime-packages,libremesh/lime-packages