Support setting devicetree for systemd-boot via bootspec

[Eisfunke]

Description of changes

This adds support for setting the devicetree when using systemd-boot as bootloader via a bootspec extension.

I'm currently running this successfully on a Raspberry Pi 4B.

Note that I've added the devicetree setting to bootspec as an extension, it might make more sense to add it natively, I'm not sure.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 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
• Fits CONTRIBUTING.md.

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[JulienMalka]

I think it makes sense to have this as a bootspec extension, it is planned to have it in bootspec V2 anyway. Will do a more complete review asap.

[JulienMalka]

Related to #319422

[Eisfunke]

Related to #319422

Oops. Didn't notice someone already did basically the same thing.

[JulienMalka]

#319422 has been merged, closing. Thank you !