boot.kernelPackages = pkgs.linuxPackages;

  # ...


  nixpkgs = {

    overlays = [
      (self: super: {
        linuxPackages = super.linuxPackages // (
          let vbox = super.callPackage ./vbox-guest.nix { kernel = super.linuxPackages.kernel; };
        in
          {
            virtualboxGuestAdditions = vbox;
            kernel = super.linuxPackages.kernel // {virtualboxGuestAdditions = vbox;};
          });
      })
      ];
  };