Set GPIO15 to OD mode in m5stack.cpp

if use M5GO button, need set gpio15 OD or PP mode to avoid affecting the wifi signal

src/M5Stack.cpp

@@ -46,6 +46,9 @@ void M5Stack::begin(bool LCDEnable, bool SDEnable, bool SerialEnable, bool I2CEn
   if (SerialEnable == true) {
     Serial.println("OK");
   }
+
+  // if use M5GO button, need set gpio15 OD or PP mode to avoid affecting the wifi signal  
+  pinMode(15, OUTPUT_OPEN_DRAIN);
 }
 
 void M5Stack::update() {