turris-maintain: config-{backup,restore}.sh: refer to tar by absolute path
Relative path to tar in config-backup.sh leads to calling busybox tar instead of gnu tar and crashing on unsupported -r option.
This reverts change to relative tar path from 71d77edf in favour of using current Turris OS absolute path.
config-restore.sh script was updated accordingly to maintain consistency.
Closes #869 (closed).
Edited by Simon Borek