Move the flush from Stream to Print

teensy3/Print.h

 	virtual size_t write(uint8_t b) = 0;
 	size_t write(const char *str)			{ return write((const uint8_t *)str, strlen(str)); }
 	virtual size_t write(const uint8_t *buffer, size_t size);
+	virtual void flush()				{ }
 	size_t write(const char *buffer, size_t size)	{ return write((const uint8_t *)buffer, size); }
 	size_t print(const String &s);
 	size_t print(char c)				{ return write((uint8_t)c); }

teensy3/Stream.h

 	virtual int available() = 0;
 	virtual int read() = 0;
 	virtual int peek() = 0;
-	virtual void flush() = 0;
 
 	void setTimeout(unsigned long timeout);
 	bool find(const char *target);