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.
espressif#1306

Author: sticilface

Diff for: libraries/WiFi/src/WiFiGeneric.cpp

@@ -147,6 +147,10 @@ static bool espWiFiStart(){
         return false;
     }
     _esp_wifi_started = true;
+    system_event_t event; 
+    event.event_id = SYSTEM_EVENT_WIFI_READY; 
+    WiFiGenericClass::_eventCallback(nullptr, &event);
+
     return true;
 }
 
@@ -369,23 +373,30 @@ void WiFiGenericClass::persistent(bool persistent)
  */
 bool WiFiGenericClass::mode(wifi_mode_t m)
 {
+    if (!_esp_wifi_started) {
+         wifiLowLevelInit(); 
+    }
     wifi_mode_t cm = getMode();
     if(cm == WIFI_MODE_MAX){
         return false;
     }
     if(cm == m) {
         return true;
     }
+    if(m){
+        espWiFiStart();
+    } else {
+        return espWiFiStop();
+    }
+    
     esp_err_t err;
     err = esp_wifi_set_mode(m);
     if(err){
         log_e("Could not set mode! %u", err);
         return false;
     }
-    if(m){
-        return espWiFiStart();
-    }
-    return espWiFiStop();
+    return true; 
+
 }
 
 /**
@@ -394,8 +405,8 @@ bool WiFiGenericClass::mode(wifi_mode_t m)
  */
 wifi_mode_t WiFiGenericClass::getMode()
 {
-    if(!wifiLowLevelInit()){
-        return WIFI_MODE_MAX;
+    if(!_esp_wifi_started){
+        return WIFI_MODE_NULL;
     }
     uint8_t mode;
     esp_wifi_get_mode((wifi_mode_t*)&mode);