turris-maintain: config-{backup,restore}.sh: refer to tar by absolute path

Simon Borekrequested to merge
hotfix/config-backup_absolute-tar-path into master

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

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