fix kconfig override

the default kconfig should be in a config stanza, not in the
default attr of the options stanza, otherwise it gets overridden
completely instead of merged

make-kernel.nix

@@ -24,6 +24,6 @@ in
 {
   vmlinux = callPackage ./make-vmlinux.nix {
     inherit tree;
-    inherit (config.kernel) config;# checkedConfig;
+    inherit config;
   };
 }