Reported by Nikolaus Schaller, Mar 24, 2017

allow to skip the partitioning and formatting step.
Optionally even one of both steps.

Then, the given partitions (chosen by -f) should be (optionally) 
wiped out and reloaded.

This should allow to e.g. update or add just a single rootfs making 
the makesd script even more a swiss army knife.

A problem might be if we want to make 3 partitions out of 2... Or 
want to allow changing partition sizes. This also seems to be 
doable, but quite tricky to safely implement.