There is an enormous practical consideration stopping the licensing change. u-boot has not required copyright assignment. This means that every single person that has contributed code to u-boot needs to give their permission for the change. This includes the authors of code copied from the Linux kernel. It isn't good enough to just post to the email list and ask for the change, you have to make sure that every single person has been asked even if they aren't paying attention to the list. If any of these people object (and some already have objected) their contribution will need to be identified, removed and rewritten.
This is not a comment on the merits or faults of the GPL v3. It is just an assessment of the logistical effort required to do the change. IMHO it would be easier to just write a new boot loader from scratch, require copyright assignment on contributions and start off with the GPL v3 initially.