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.
```
$ disk_layout="ada2 ada1 ada0"
$ echo ${disk_layout##*[1-9]} | cut -d' ' -f1
ada0
$ disk_layout="da0 ada0 vtbd0"
$ echo ${disk_layout##*[1-9]} | cut -d' ' -f1
da0
```

Now I don't know freebsd well enough to know what's desired here, but I'm guessing it's better than just removing the spaces.