This makes it look a bit more like other tests and also prepares the
addition of further test cases and different tests.

All board specific quirks inside the installer (and other bugs) relating to
root on softraid are fixed, so that install media with the new question
just works for plain as well as encrypted installs.

Tested on Pinebook Pro, Raspberry Pi 4b, SolidRun CEX7.

Various files are put onto a pi's ESP, assuming root disk means boot disk,
which is not true for root on softraid(4).

Wrap the code in a loop over chunks (if any) or the single disk as before,
just like installboot(8) does.

OK mbuhl