Wait for root mount tokens before showing the root mount prompt.
This restores the pre-r290196 behaviour, eliminating the need to manually
press '.' a couple of times to get USB to finish probing.

Note that there's still something wrong with the console (character
echoing doesn't quite work), and there's also a reported problem with
BHyVe, but those two don't seem related to the problem above.

MFC after: 1 month
Sponsored by: The FreeBSD Foundation