FreeBSD: do not hardcode disk device

Replace the hardcoded disk device 'ada0' with a more dynamic approach.

It's based on an idea [by Amitabh Kant](http://www.amitabhkant.com/partition-automation-in-bsdinstall-auto-script-in-freebsd/), so credits go to him.
Yes, but I think now that this re-introduces the behaviour from the beginning with e.g. ada2 being used instead of ada0.