add default role to system

main/boards/atoms3r-echo-base/atoms3r_echo_base.cc

@@ -160,7 +160,7 @@ private:
         InitializeButtons();
         display_->SetStatus("错误");
         display_->SetEmotion("sad");
-        display_->SetChatMessage("", "Echo Base\n未连接");
+        display_->SetChatMessage("system", "Echo Base\n未连接");
         
         while (1) {
             ESP_LOGE(TAG, "Atomic Echo Base is disconnected");