Build system integration for U-boot on Mocha

[ziuziakowska]

CMake integration for building U-boot for Mocha from a fork with support.

U-boot should be built with a Mocha defconfig file, and then wrapped as a payload for OpenSBI.

[ziuziakowska]

Have this implemented locally, as well as the necessary devicetree changes. Just needs some cleanup.

[ziuziakowska]

Closed by #578.