Failure fix/reconnecting

[QuangNguyen1412]

Hi Tom,

        if (wifi_manager_check_connection() == ERR_WIFI_DISCONECTED) {
            wifi_manager_fetch_wifi_sta_config();
            if(strlen((char*)wifi_manager_config_sta->sta.ssid) > 0) {
                wifi_manager_connect_async();
            } else {
                ESP_LOGW("%s:[%d] - Not going to automatically connect to wifi");
                // Not going retry since... esp would stop working until the next reboot
                // Do something else but retry connecting
            }
        }

this is the code that I added to solve 2 things:

• Calling to wifi_manager_fetch_wifi_sta_config() to fetch saved data to the containing structure. otherwise it would not work, we cant even continue with the configuration webpage.
• the conditional snippet to prevent the same case on booting up. Without any save wifi configuration, we will not attempt to connect to a null value

Could you take a look, review and merge into master if it would not fail other cases?

[QuangNguyen1412]

i also added MQTT AirU configuration to Readme in case anyone else may need it later (other research group?). But I think leaving the mqtt_id and password is not a good idea, I'll let you decide