Motivation for this change

Keeping untested, and untestable packages around is not great.

As maintainer of U-Boot-related things, I want to remove the last non-mainline U-Boot from Nixpkgs.

All mainline-based U-Boots are basically free, assuming that mainline keeps them in working order.

From this point forward, it would be ideal to not add non-mainline U-Boot to Nixpkgs. Let's see if we can do it.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

cc @viric who introduced and maintained the package.