Added missing begin() variant with 2nd argument for configuring serial port (parity etc..)

cores/arduino/HardwareSerial.h

@@ -68,6 +68,7 @@ class HardwareSerial : public Stream
 {
   public:
     virtual void begin(unsigned long);
+    virtual void begin(unsigned long baudrate, uint8_t config);
     virtual void end();
     virtual int available(void) = 0;
     virtual int peek(void) = 0;