uboot: sheevplug and guruplug: use mainline U-Boot

These derivations have not seen any updates since they were created in 2010,
and some of their sources have disappeared. There are upstream configs for
these boards, so these are now used, and they build correctly. I have no way
of testing them, and I don't if anyone even uses either board with Nix anymore.

(cherry picked from commit 01020b32)
(cherry picked from commit 48ade50d8ece09d3ff732b07f0facdcd78084ac3)