Fix aufs utilities for NixOS manual

svn path=/nixpkgs/trunk/; revision=17153

pkgs/os-specific/linux/aufs2-utils/default.nix

@@ -1,6 +1,4 @@
 a :  
-if (a.lib.attrByPath ["features" "zen"] false a.kernel) ||
-  (a.lib.attrByPath ["features" "aufs"] false a.kernel) then
 let 
   s = import ./src-for-default.nix;
   buildInputs = with a; [
@@ -35,4 +33,3 @@ rec {
       linux;
   };
 }
-else throw "Kernel doesn't support AUFS"

pkgs/os-specific/linux/zen-kernel/2.6.31-zen0.nix

@@ -12,6 +12,7 @@ in
     iwlwifi = true;
     zen = true;
     fbConDecor = true;
+    aufs = true;
   };
 
   extraMeta = {

pkgs/top-level/all-packages.nix

@@ -5357,9 +5357,11 @@ let
       inherit fetchgit stdenv kernel perl;
     };
 
-    aufs2Utils = builderDefsPackage ../os-specific/linux/aufs2-utils {
-      inherit kernel;
-    };
+    aufs2Utils = if kernel.features ? aufs then
+      builderDefsPackage ../os-specific/linux/aufs2-utils {
+        inherit kernel;
+      } 
+    else null;
 
     exmap = import ../os-specific/linux/exmap {
       inherit fetchurl stdenv kernel boost pcre pkgconfig;