This is due to us many times (re-)using Linux drivers inside U-Boot.
This won't stop you from making sure all of U-Boot (aside from these drivers) says "GPLv2 or later". Also, you can talk with the developers of the drivers that you need, or might need, to ask them to release their drivers GPLv2+.
The reasoning seems to be that companies using U-Boot inside a commercial product consider it to be "a neccessary precondition to only accept blessed firmware upgrades" (my wording).
What they mean is they think these companies want to tivoize it. I don't know whether that is true. For discussion, let's suppose it is true.
It has no impact whatsoever on making U-boot support GPLv2+.
However, it means that moving to GPLv3+ means a decision about values. Is it good for companies to use your program to deny the users' freedom? Or is it better to defend users' freedom by saying no to those companies? If a company seeks softwart with which to restrict users, would you rather tell them "Please do it with my program" or "I don't want to share the responsibility for what you are doing?"
Is popularity the most important value?
We should also start to actively inform the regularly appearing people on this mailing list complaining that they cannot get the source code to U-Boot of "device xyz" that with a GPLv2 U-Boot this may become a theoretical question in the future when they cannot install the changed binary anymore.
Exactly!