boot.kernelPackages: check for conflicts

It's currently possible to set conflicting `boot.kernelPackages` several times
which can prove confusing.
This is an attempt to warn for this.

nixos/modules/system/boot/kernel.nix

@@ -36,6 +36,7 @@ in
 
     boot.kernelPackages = mkOption {
       default = pkgs.linuxPackages;
+      type = types.unspecified // { merge = mergeEqualOption; };
       apply = kernelPackages: kernelPackages.extend (self: super: {
         kernel = super.kernel.override {
           inherit randstructSeed;