This adds support for the Radxa Rock Pi 4 board which is based on
RK3399. The u-boot outputs from the introduced build are suitable to
build a generic image for the Rock Pi 4 by setting the following in a
custom build of $nixpkgs/nixos/modules/installer/cd-dvd/sd-image-aarch64.nix:
```
sdImage.postBuildCommands = ''
dd if=${pkgs.ubootRockPi4}/idbloader.img of=$img seek=64 conv=notrunc
dd if=${pkgs.ubootROckPi4}/u-boot.itb of=$img seek=16384 conv=notrunc
'';
```
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