Simplify support for flash-kernel based boards

We want to use automatic uboot setup via flash-kernel so make that the default instead of coding around it various places. It also makes /boot way smaller on the devkit.

@bob.ham it's be cool if we could switch over the nitrogen board too, i'll look at the NXP EVK once I need to reflash it again.

Marking as wip until i completed a full test cycle on the devkit.