[core] Organize GCC compilation flags

cores/common/arduino/libraries/common/IPv6Address/IPv6Address.cpp

@@ -54,6 +54,7 @@ size_t IPv6Address::printTo(Print &p) const {
 
 	}
 	return n; */
+	return 0;
 }
 
 String IPv6Address::toString() const {

cores/common/arduino/libraries/common/Update/Update.h

@@ -104,6 +104,7 @@ class UpdateClass {
   public:
 	String md5String(void) {
 		// return _md5.toString();
+		return "";
 	}
 
 	void md5(uint8_t *result) {

cores/common/arduino/libraries/common/WiFiServer/LwIPServer.cpp

@@ -61,6 +61,7 @@ bool LwIPServer::begin(uint16_t port, bool reuseAddr) {
 	_active		  = true;
 	_noDelay	  = false;
 	_sockAccepted = -1;
+	return true;
 }
 
 void LwIPServer::end() {