kernelExtraConfig

# In configuration.nix:

nixpkgs.config = {
    packageOverrides = pkgs: {
      stdenv = pkgs.stdenv // {
        platform = pkgs.stdenv.platform // {
          kernelExtraConfig = ''
	    CONFIG_EXTRA_FIRMWARE_DIR \"/lib/firmware\"
	  '';
        };
      };
    };
  };

# Fails with:

GOT: # configuration written to .config
GOT: #
GOT: make[1]: Leaving directory '/tmp/nix-build-linux-config-3.18.21.drv-0/build'
GOT: make: Leaving directory '/tmp/nix-build-linux-config-3.18.21.drv-0/linux-3.18.21'
unused option: CONFIG_EXTRA_FIRMWARE
builder for ‘/nix/store/xbns29cnhcisv7k34l7w5b4diqy5xj0l-linux-config-3.18.21.drv’ failed with exit code 255
cannot build derivation ‘/nix/store/2762nvgdaylzqga7h0qp4rji7g0s4ai0-linux-3.18.21.drv’: 1 dependencies couldn't be built
cannot build derivation ‘/nix/store/bbizczsg63rrmfhzhvbfdcdb75mpvdmh-nixos-system-lambda-16.03pre69463.68bd8e4.drv’: 1 dependencies couldn't be built
error: build of ‘/nix/store/bbizczsg63rrmfhzhvbfdcdb75mpvdmh-nixos-system-lambda-16.03pre69463.68bd8e4.drv’ failed
[100] barton@lambda>