Add option to select the core used for Arduino and it's events

cores/esp32/HardwareSerial.h

@@ -113,6 +113,9 @@ protected:
 extern void serialEventRun(void) __attribute__((weak));
 
 #if !defined(NO_GLOBAL_INSTANCES) && !defined(NO_GLOBAL_SERIAL)
+#ifndef ARDUINO_SERIAL_PORT
+#define ARDUINO_SERIAL_PORT 0
+#endif
 #if ARDUINO_SERIAL_PORT //Serial used for USB CDC
 #include "USB.h"
 #include "USBCDC.h"