Werk #9545: Migrate /rw from aufs to overlayfs

The OS overlay filesystem is migrated from aufs to overlayfs with this version. This change is a necessary preparation for the upcoming upgrade of the firmware to Debian 12.

The transition to OverlayFS is performed automatically during firmware upgrade.

Details:

The storage of the appliance is separated into two areas. The operating system with it's configuration and the Checkmk sites with the installed versions. This change is about the first area.

This operating system area is realized by a layered filesystem, where the firmware we deploy is the foundation, the lower layer. This layer is read-only. The upper layer can be written and overlays the firmware. This allows the user to put own files into the root filesystem and modify or delete files we ship with the firmware. The files that are created in the root filesystem during runtime are written to the upper layer. This is automatically managed by the overlay filesystem driver. The user does not have to do anything special for that.

Up to 1.6.x the overlay filesystem was realized using aufs (https://en.wikipedia.org/wiki/Aufs) which served us well throughout the years. However, it is not available anymore for newer Linux kernel versions. It's functionality is provided by OverlayFS (https://en.wikipedia.org/wiki/OverlayFS).

The migration is being performed automatically during the early phase of the first startup after upgrading the appliance to 1.7.0 or newer. The migration writes log entries to TTY2 (CTRL+ALT+F2) during the startup. The output of the script is also made available for later review at /var/log/cma-migrate-overlayfs.log.

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