Fix grub error "cp cannot stat .... unicode.pf2

-- change postPatch hook to prePatch (postPatch is not run with empty patchset)
-- make the share/grub directory and copy unicode.pf2 there

svn path=/nixpkgs/trunk/; revision=26239

pkgs/tools/misc/grub/1.9x.nix

@@ -41,12 +41,17 @@ stdenv.mkDerivation rec {
            -e's/qemu-system-i386/qemu-system-x86_64 -nodefaults/g'
     '';
 
-  postPatch =
+  prePatch =
     '' gunzip < "${unifont_bdf}" > "unifont.bdf"
        sed -i "configure" \
            -e "s|/usr/src/unifont.bdf|$PWD/unifont.bdf|g"
     '';
 
+  postInstall = ''
+    ensureDir ./share/grub
+    cp ./unicode.pf2 ./share/grub/ 
+  '';
+
   configureFlags =
     let arch = if stdenv.system == "i686-linux" then "i386"
                else if stdenv.system == "x86_64-linux" then "x86_64"