Make root on softraid installations boot out of the box on Raspberry Pis

Make root on softraid installations boot out of the box on Raspberry Pis

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