fix kernel 3.4+ early cifs mounting (qemu-vm target)

kernel 3.4+ needs cifs-utils to mount CIFS filesystems.
the kernel itself (and busybox's cifs mount code) are no longer able
to do this in some/most cases and will error out saying:
"CIFS VFS: connecting to DFS root not implemented yet"

Nixos' qemu-vm target is hurt by this, as it wants to mount /nix/store
via cifs very early in the boot process.

This commit makes sure the initrd for affected kernels is built with
cifs-utils if needed.

modules/system/boot/stage-1.nix

@@ -121,6 +121,20 @@ let
   enableSplashScreen =
     config.boot.vesa && config.boot.initrd.enableSplashScreen && kernelPackages.splashutils != null;
 
+  needsCifsUtils = kernelPackages.kernel ? features
+                && kernelPackages.kernel.features ? needsCifsUtils
+                && kernelPackages.kernel.features.needsCifsUtils
+                && any (fs: fs.fsType == "cifs") fileSystems;
+
+  busybox = if needsCifsUtils
+            then pkgs.busybox.override {
+                   extraConfig = ''
+                     CONFIG_FEATURE_MOUNT_CIFS n
+                     CONFIG_FEATURE_MOUNT_HELPERS y
+                   '';
+                 }
+            else pkgs.busybox;
+
 
   # Some additional utilities needed in stage 1, like mount, lvm, fsck
   # etc.  We don't want to bring in all of those packages, so we just
@@ -146,7 +160,7 @@ let
       cp -pv ${pkgs.gcc.gcc}/lib*/libgcc_s.so.* $out/lib
 
       # Copy BusyBox.
-      cp -rvd ${pkgs.busybox}/{bin,sbin} $out/
+      cp -rvd ${busybox}/{bin,sbin} $out/
       chmod -R u+w $out
 
       # Copy some utillinux stuff.
@@ -175,6 +189,11 @@ let
         cp ${kernelPackages.splashutils}/${kernelPackages.splashutils.helperName} $out/bin/splash_helper
       ''}
 
+      # Maybe copy cifs utils
+      ${optionalString needsCifsUtils ''
+        cp -v ${pkgs.cifs_utils}/sbin/mount.cifs $out/bin
+      ''}
+
       ${config.boot.initrd.extraUtilsCommands}
 
       # Strip binaries further than normal.