[shill] Wire up derived properties in the Device class

BUG=chromium-os:16343
TEST=unit tests

Change-Id: Ib35efbc00133b6ffc9352e52c0413be4bd984e12
Reviewed-on: http://gerrit.chromium.org/gerrit/3559
Tested-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>
diff --git a/ipconfig.h b/ipconfig.h
index cd9e8fd..08915e3 100644
--- a/ipconfig.h
+++ b/ipconfig.h
@@ -52,6 +52,8 @@
 
   const std::string &device_name() const { return device_name_; }
 
+  std::string GetRpcIdentifier();
+
   // Registers a callback that's executed every time the configuration
   // properties change. Takes ownership of |callback|. Pass NULL to remove a
   // callback. The callback's first argument is a pointer to this IP