HALFARSED enable toggling promptless kconfig symbols

Some kernel config symbols we need to enable in some configurations
don't have a "prompt" entry in the Kconfig files, meaning that
make oldconfig (or any other make fooconfig) won't change them.
So we add a Kconfig.local where they can be selected

In this patch we hardcode the required symbol. Next step would be
to generalise so that they could be specified in something like a
config.kernel.hiddenConfig attrset

kernel/default.nix

@@ -25,7 +25,8 @@ let
   tree = stdenvNoCC.mkDerivation {
     name = "spindled-kernel-tree";
     src = source;
-    phases = [ "unpackPhase" "patchScripts" "installPhase" ];
+    phases = [ "unpackPhase" "patchPhase" "patchScripts" "installPhase" ];
+    patches = [ ./random.patch ];
     patchScripts = ''
       patchShebangs scripts/
     '';