U-boot's license requires you to provide the source-code for U-boot (including any modifications you made) to anybody that asks for it. U-boot typically doesn't interact with your system after booting, and neither will its license.
In general, GPLv2 software is OK to ship on a product that also includes closed-source software. It won't force you to give away your proprietary code, as long as all you're doing is running GPLv2 binaries or dynamically linking against GPLv2 libraries (note: the topic of linking is up for some interpretation).
GPLv3 software is a bit stricter, and requires you to provide users with a way to modify the filesystem that holds any GPLv3 code. So if you need to have encrypted updates or something, try to steer clear of GPLv3 software. But GPLv2 is usually OK as long as you're OK giving out any modifications you made.