WIP: hack to use system_layout.json for mounting /data

Not really intuitive to have data mounted by a script called 71_iopsys_overlay and abusing it. In addition /data sits alongside the other partitions in the config-file that are related to A/B. I wonder whether we can design a more generic solution. Maybe by working with suffixes for A/B slot and making that optional, so that if that is not defined, the partition belongs to all slots? Or a different section in the file?

Needs to be refactored somehow to find an easy to use and easy to maintain solution.