[USART] Improved clearQueue method for flushing the internal receiver queue

Closes issue #120 (closed). Now the clearQueue accounts also for ongoing transactions. We start by flushing the receiver queue and then we read all the incoming message. In this way we won't have invalid messages in the queue

src/shared/drivers/usart/USART.cpp

@@ -595,7 +595,14 @@ void USART::writeString(const char *buffer)
     };
 }
 
-void USART::clearQueue() { rxQueue.reset(); }
+void USART::clearQueue()
+{
+    char buf[usart_queue_default_capacity];
+    rxQueue.reset();
+    while (read(buf, usart_queue_default_capacity))
+        ;
+    rxQueue.reset();
+}
 
 STM32SerialWrapper::STM32SerialWrapper(USARTType *usart, int baudrate)
     : USARTInterface(usart, baudrate)