Remove busy loop from rx_thread

[trivialkettle]

The rx_thread executed a busy loop and loaded a single core to 100%. This change reduces the load significantly.

There would also be another solution by using

        """ process data as long as requested """
        while self.rx_thread_state == USBtin.RX_THREAD_RUNNING:
            buf = self.serial_port.read(max(1, self.serial_port.inWaiting()))
            if buf:
# ...

This read blocks until 1 or inWaiting() bytes are available. Additionally one need to update stop_rx_thread() by adding

        self.serial_port.cancel_read()

to cancel the blocking read.

[trivialkettle]

I created a pull request with my other proposed changes in #19 after testing this.