Fix N9 boot.

Without this, sdcard fails to start because its mount point doesn't
exist.

Bug: http://b/22568628
Change-Id: Ic6a2f51fc74c5bd1f23d97592049495ac3822bb3
diff --git a/rootdir/init.rc b/rootdir/init.rc
index be5292a..b7a593f 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -17,6 +17,9 @@
     # Set the security context of /adb_keys if present.
     restorecon /adb_keys
 
+    # Shouldn't be necessary, but sdcard won't start without it. http://b/22568628.
+    mkdir /mnt 0775 root system
+
     start ueventd
 
 on init