router short address set to 0xfffe for non routers (ARMmbed#1543)

upon receiving child id response. This prevents the
REED from requesting a child address when trying to
upgrade.

Author: deepakvenugopal

source/6LoWPAN/Thread/thread_host_bootstrap.c

@@ -835,6 +835,9 @@ static void thread_mle_child_request_receive_cb(int8_t interface_id, mle_message
             thread_info(cur)->thread_attached_state = THREAD_STATE_CONNECTED;
 
             thread_bootstrap_update_ml16_address(cur, childId);
+            if (!thread_is_router_addr(thread_info(cur)->routerShortAddress)) {
+                thread_info(cur)->routerShortAddress = 0xfffe;
+            }
 
             mle_service_msg_free(scan_result->child_id_request_id);