Move U-Boot packages from temp into device

I'm expecting this to be at least somewhat controversial, and thus the request for comments mark in the title. My rationale for this is that these U-Boot forks really aren't that temporary (and never existed upstream in Alpine) and they are all at least somewhat device-specific. We already have some U-Boot forks inside of device, so let's be consistent about it. This also cleans up temp significantly and makes it easier to focus on the more actionable packages.