lib.kernel.unset: init

Previously, there was no way to unset an option when overriding a
kernel, apart from writing out the attrset yourself.  Now it's
possible with lib.mkForce lib.kernel.unset.  It's important to be able
to do this, because setting an option in the override may cause other
options to become unused, which would fail the config build unless
they were overridden too.

lib/kernel.nix

@@ -8,9 +8,10 @@ with lib;
   option = x:
       x // { optional = true; };
 
-  yes      = { tristate    = "y"; optional = false; };
-  no       = { tristate    = "n"; optional = false; };
-  module   = { tristate    = "m"; optional = false; };
+  yes      = { tristate    = "y";  optional = false; };
+  no       = { tristate    = "n";  optional = false; };
+  module   = { tristate    = "m";  optional = false; };
+  unset    = { tristate    = null; optional = false; };
   freeform = x: { freeform = x; optional = false; };
 
   /*