Allow User to manage ESP8266 Wifi if need

If MY_ESP8266_SSID is not defined, the WIFI part is managed by the user.
If MY_ESP8266_SSID is defined, wifi is managed by Mysensor libraries.

libraries/MySensors/core/MyGatewayTransportEthernet.cpp

@@ -100,6 +100,7 @@ typedef struct
 bool gatewayTransportInit() {
 	_w5100_spi_en(true);
 	#if defined(MY_GATEWAY_ESP8266)
+		#if defined(MY_ESP8266_SSID)
 			(void)WiFi.begin(MY_ESP8266_SSID, MY_ESP8266_PASSWORD);
 			#ifdef MY_IP_ADDRESS
 				WiFi.config(_ethernetGatewayIP, gateway, subnet);
@@ -112,6 +113,7 @@ bool gatewayTransportInit() {
 			}
 			MY_SERIALDEVICE.print(F("IP: "));
 			MY_SERIALDEVICE.println(WiFi.localIP());
+		#endif
 
 	#else
 		#ifdef MY_IP_ADDRESS