nixos/modules/system/boot/loader/raspberrypi: enable u-boot support for rpi 4

[uninsane]

Description of changes

allows installation of u-boot to the /boot directory of a rpi4 system via

boot.loader.raspberryPi = {
  enable = true;
  uboot.enable = true;
  version = 4;
};

whereas previously version = 4 would have triggered an assertion in the derivation.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.11 Release Notes (or backporting 22.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

---

i've been using this for about a month on the raspberry pi 400, which uses the same SOC as the rpi4.
i haven't tested the 32-bit code path.

it's not clear to me why support was previously disabled at this level: i think it's just an artifact of history.
the nixos SD image gained support for rpi4 in cc5baf2 (2019/08/17).
that commit added the branch which throws "U-Boot is not yet supported on the raspberry pi 4" (which my patch removes).
nixpkgs u-boot gained rpi4 support in 95cd8c8 (2020/09/12).
i think at that point we could have enabled rpi support here but it was overlooked.

tagging @tkerber for his earlier involvement in this area.

[astro]

See #112677

[uninsane]

closing in favor of the PR linked above, which accomplishes everything here but probably handles the armstubs more reliably. thanks for digging that up, astro