[coreboot] Returning control to v3 from the payload

[Jordan Crouse]

Somewhere in the back of my mind, I think I recall it was a design
goal that we support returning control back to v3 from the payload
and loading an alternate payload or panicing sanely.

Is this still a legitimate goal for v3?  If so, how much of the original
environment needs to be restored before the payload returns control?
I ask because I'm finishing up the last touches of the libpayload
BSD rewrite, and this is really the last thing I need to worry about.

Jordan