I think this would be a good move for interoperability, so that GPLv3 projects (e.g. SeaBIOS) may take code from coreboot where needed (e.g. device drivers). I'm strictly talking about listing the preferred license; it's still up to contributors whether they want v2 or v2+.
[1] Listed GPLv2+ as preferred, but it was recently changed without any explanation. Stefan, do you have more details on that?
Alex
[1] https://www.coreboot.org/Development_Guidelines#Common_License_Header