I discovered a problem in the file drivers/serial/serial.c concerning the...

I discovered a problem in the file drivers/serial/serial.c concerning the function uart_close(…). In the case that a serial device is opened with the flag O_NONBLOCK the function uart_close(…) blocks until all data in the buffer is transmitted. The function close(…) called on an handle opened with O_NONBLOCK should not block. The problem occurred with a CDC/ACM device.