nixos/initrd: add extraFirmwarePaths option

[jmbaur]

Some platforms don't specify firmware needed by kernel drivers in the driver code itself (i.e. query-able with modinfo). Platforms that use devicetree often specify firmware in the devicetree itself (to see what boards do this from a linux source tree, run grep -r 'firmware-name =' arch/*/boot/dts). Since our existing initrd builder cannot auto-detect all firmware to include in this case outlined above, we should provide a way to specify extra firmware files to include in the initrd.

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/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 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]

Otherwise, LGTM