Change robot_base_frame in kobuki_gazebo.urdf.xacro file.

Before the fix, the tf tree should look like base_footprint -> base_link -> ...
When simulating Kobuki using Gazebo, the plugin would publish: odom -> base_link
This caused an error in the transform from base_footprint to base_link since one link could only have one parent.

To prevent the Gazebo plugin (diff-drive) from publishing odom -> base_link, we should modify the configuration in the URDF.

Reference: kobuki-base/kobuki_ros#49

kobuki_ws/src/kobuki_ros/kobuki_description/urdf/kobuki_gazebo.urdf.xacro

@@ -193,7 +193,7 @@
         <publish_wheel_tf>true</publish_wheel_tf>
         <odometry_topic>odom</odometry_topic>
         <odometry_frame>odom</odometry_frame>
-        <robot_base_frame>base_link</robot_base_frame>
+        <robot_base_frame>base_footprint</robot_base_frame>
 	    </plugin>
 	  </gazebo>
   </xacro:macro>