Remove Serial definition if not needed

cores/esp8266/HardwareSerial.h

@@ -233,8 +233,12 @@ class HardwareSerial: public Stream
  size_t _rx_size;
 };
 
+#if !defined(NO_GLOBAL_INSTANCES) && !defined(NO_GLOBAL_SERIAL)
 extern HardwareSerial Serial;
+#endif
+#if !defined(NO_GLOBAL_INSTANCES) && !defined(NO_GLOBAL_SERIAL1)
 extern HardwareSerial Serial1;
+#endif
 
 extern void serialEventRun(void) __attribute__((weak));