Fix SerialBT.flush (#3579)

* Fix flush

flush should not flush the read buffer, it should wait till all output is drained from the write buffer.

* Update BluetoothSerial.cpp
Co-authored-by: NMe No Dev <me-no-dev@users.noreply.github.com>

libraries/BluetoothSerial/src/BluetoothSerial.cpp

@@ -660,7 +660,11 @@ size_t BluetoothSerial::write(const uint8_t *buffer, size_t size)
 
 void BluetoothSerial::flush()
 {
-    while(read() >= 0){}
+    if (_spp_tx_queue != NULL){
+        while(uxQueueMessagesWaiting(_spp_tx_queue) > 0){
+	    delay(5);
+        }
+    }
 }
 
 void BluetoothSerial::end()