hack: support 'devicetree' with systemd-boot + hardware.device-tree

[colemickens]

Description of changes

1. Hack the toplevel derivation to output a /dtb symlink if it's enabled/configured with hardware.device-tree.{enable,name}.
2. Hack the systemd-boot-builder.py to output the extra appropriate devicetree= line into the config file if it exists in the generation toplevel.

I'm not sure if this supports specialisations. The more appropriate path probably involves trying to get this into bootspec v2 and lanzaboote, but this was easier for now.

I've used this to build make-disk-image.nix for a snapdragon X elite laptop, and booted into initrd (pending stage-2 when some usb/msd/uas issue sorted).

Mostly throwing this out for consideration, I realize it's probably too hacky (and certainlyuntested) to merge as-is. Marking as draft to reflect that.

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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[ElvishJerricco]

Other implementations of the same: #319422, #330195

[colemickens]

Oh wow, and they both went ahead and used bootspec extensions. Nice. Good work to those authors. Hopefully one takes off. I'll try to review them more, but probably needs feedback from bootspec folks.