add flush functions for serial helpers

src/fsfw_hal/linux/serial/helper.cpp

@@ -161,3 +161,7 @@ void uart::setStopbits(struct termios& options, StopBits bits) {
     options.c_cflag &= ~CSTOPB;
   }
 }
+
+void uart::flushRxBuf(int fd) { tcflush(fd, TCIFLUSH); }
+
+void uart::flushTxRxBuf(int fd) { tcflush(fd, TCIOFLUSH); }