add option to Flush() to only clear txQueue

Add the option to cause Flush() to just wait for tx data to clear the tx fifo and uart, leave the rx queue and rx fifo as is.

Author: stickbreaker

cores/esp32/HardwareSerial.h

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