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.
OK, so it seems my first guess was correct... @fraenki please do ;)