Add config option ‘allowUnfree’

If set to false, mkDerivation will throw an exception if a package has
an unfree license.  ‘release-lib.nix’ uses this to enforce that we
don't build unfree packages as part of the Nixpkgs channel.  Since
this is set through Nixpkgs' ‘config’ argument, it's more finegrained
than $HYDRA_DISALLOW_UNFREE.

pkgs/os-specific/linux/aufs/3.nix

@@ -33,6 +33,7 @@ stdenv.mkDerivation {
     '';
 
   passthru = { inherit patch; };
+
   meta = {
     description = "Another Unionfs implementation for Linux (third generation)";
     homepage = http://aufs.sourceforge.net/;