Implement Tx only Flush (#3433)

* 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.

* support tx only flush()

* support tx only Flush()

* support txOnly for Flush()

* compatibility to Stream()

* compatibility for Stream()

* default value error

* default value error

* Update esp32-hal-uart.h

* Update esp32-hal-uart.c

* Update HardwareSerial.cpp

* sp

* correctly implement flushTxOnly()

cores/esp32/HardwareSerial.h

@@ -63,6 +63,7 @@ public:
     int peek(void);
     int read(void);
     void flush(void);
+    void flush( bool txOnly);
     size_t write(uint8_t);
     size_t write(const uint8_t *buffer, size_t size);