Thanks JPMonteiro.
That would be a very handy thing to have. Most Unix devices going back to early Sun workstations had boot into single user mode. It comes in handy if there's a regular startup problem so you can edit startup config files in root and bring things back up.
In my case I was stuck in a startup loop and couldn't get to Unix prompt. Ended up having to re-flash the whole thing and had to go back to zero.