include core_esp8266_features.h in arduino.h for easier usage, and add WIFI_HAS_EVENT_CALLBACK

Author: Links2004

cores/esp8266/Arduino.h

@@ -37,6 +37,7 @@ extern "C" {
 #include "binary.h"
 #include "esp8266_peri.h"
 #include "twi.h"
+#include "core_esp8266_features.h"
 
 #define HIGH 0x1
 #define LOW  0x0

cores/esp8266/core_esp8266_features.h

@@ -28,6 +28,8 @@
 #define CORE_HAS_LIBB64
 #define CORE_HAS_BASE64_CLASS
 
+#define WIFI_HAS_EVENT_CALLBACK
+
 
 #endif