compatibility to Stream()

Author: stickbreaker

cores/esp32/HardwareSerial.h

@@ -62,7 +62,8 @@ class HardwareSerial: public Stream
     int availableForWrite(void);
     int peek(void);
     int read(void);
-    void flush( bool txOnly = false);
+    void flush(void);
+    void flush( bool txOnly);
     size_t write(uint8_t);
     size_t write(const uint8_t *buffer, size_t size);