small fixes to build custom kernel

svn path=/nixpkgs/trunk/; revision=13795

pkgs/os-specific/linux/kernel/generic.nix

@@ -34,6 +34,8 @@
 , # A list of additional statements to be appended to the
   # configuration file.
   extraConfig ? []
+
+, preConfigure ? ""
 }:
 
 assert stdenv.system == "i686-linux" || stdenv.system == "x86_64-linux";
@@ -55,6 +57,8 @@ stdenv.mkDerivation {
   
   builder = ./builder.sh;
 
+  inherit preConfigure;
+
   inherit src config;
   
   patches = map (p: p.patch) kernelPatches;