Build generic aarch64 / riscv64 images

[kallisti5]

If multiplix ever transitions to FDT's, theoretically it could build generic "aarch64" or "riscv64" images.

Haiku has a tool (full disclosure, I wrote it) for end users to add pre-compiled board-specific u-boot loaders to os images.

The u-boot binaries contain embedded FDT's for their compiled board, so you can get "all the information you need" about the running device from u-boot.