nixos: make it easy to apply kernel patches

This makes it easy to specify kernel patches:

    boot.kernelPatches = [ pkgs.kernelPatches.ubuntu_fan_4_4 ];

To make the `boot.kernelPatches` option possible, this also makes it
easy to extend and/or modify the kernel packages within a linuxPackages
set. For example:

    pkgs.linuxPackages.extend (self: super: {
      kernel = super.kernel.override {
        kernelPatches = super.kernel.kernelPatches ++ [
          pkgs.kernelPatches.ubuntu_fan_4_4
        ];
      };
    });

Closes #15095