HardwareSerial's flush() method to override the method of the base class Print (#2254)

* add flush() method to HardwareSerial

* remove default argument

Author: ilolis

cores/arduino/HardwareSerial.cpp

@@ -487,6 +487,11 @@ int HardwareSerial::availableForWrite(void)
   return tail - head - 1;
 }
 
+void HardwareSerial::flush()
+{
+  flush(0);
+}
+
 void HardwareSerial::flush(uint32_t timeout)
 {
   // If we have never written a byte, no need to flush. This special

cores/arduino/HardwareSerial.h

@@ -125,7 +125,8 @@ class HardwareSerial : public Stream {
     virtual int peek(void);
     virtual int read(void);
     int availableForWrite(void);
-    virtual void flush(uint32_t timeout = 0);
+    virtual void flush();
+    void flush(uint32_t timeout);
     virtual size_t write(uint8_t);
     inline size_t write(unsigned long n)
     {