Yes, most of this is all supported. There is a existing recipe for building an image that you just dd to and SD card and can boot in a Raspberry Pi (and other devices supported by mainline kernel + u-boot).

Kernel is a package just like another; so you just write your own recipe. Patching kernel recipe works just like patching any Nix recipe.

Not sure about the rockchip-proprietary boot parts; you may be on your own.