servo: Add the new method get_base_board() to the Servo object

This RPC is introduced in servod to get the board name of the base.

BUG=b:66211919
CQ-DEPEND=CL:675746
TEST=None, waiting a coming change to add tests to this method.

Change-Id: Iaf6d47373aa7cfa8f3d4fb3e37811f2a8d883663
Reviewed-on: https://chromium-review.googlesource.com/676229
Commit-Ready: Wai-Hong Tam <waihong@google.com>
Tested-by: Wai-Hong Tam <waihong@google.com>
Reviewed-by: Kevin Shelton <kmshelton@chromium.org>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>
diff --git a/server/cros/servo/servo.py b/server/cros/servo/servo.py
index 01f92f9..e0c268f 100644
--- a/server/cros/servo/servo.py
+++ b/server/cros/servo/servo.py
@@ -443,12 +443,15 @@
 
 
     def get_board(self):
-        """Get the board connected to servod.
-
-        """
+        """Get the board connected to servod."""
         return self._server.get_board()
 
 
+    def get_base_board(self):
+        """Get the board of the base connected to servod."""
+        return self._server.get_base_board()
+
+
     def _get_xmlrpclib_exception(self, xmlexc):
         """Get meaningful exception string from xmlrpc.