From scottr/NetBSD: Completely revamp setroot() and friends.  Since Chris' version of this code is close to how I reworked it, I pulled in the (essentially MI) NetBSD/alpha and added the appropriate support around it.
From scottr/NetBSD:
Completely revamp setroot() and friends.  Since Chris' version of this code
is close to how I reworked it, I pulled in the (essentially MI) NetBSD/alpha
and added the appropriate support around it.

- No need for the GENERIC kernel config option.
- NFS-mounted root and swap are supported.
- If we can't figure out where the root filesystem is from what the Booter
  tells us, ask the user.
- Split the mainbus autoconfig code to a separate file.

Also, update/add copyrights as appropriate.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars