* Add `SYSTEM_EVENT_WIFI_READY` call back once wifi service is init. allows you to hook in, as the sdk does not generate this event for you.
As it stands the SDK does not appear to set `WIFI_MODE_NULL` correctly. if the wifi is initialised and set to `WIFI_MODE_NULL` it actually defaults to AP mode. This fix keeps `WIFI_MODE_NULL` within the ESP class if the wifi has not been init yet, and works in my testing. albeit a one sided conversation.
https://github.com/espressif/arduino-esp32/issues/1306
* make changes compatible with new _persistent behaviour.