Semantic of mraa_uart_set_timeout with zero read timeout unclear

[jan-kiszka]

The documentation states "<= 0 will disable that timeout" (for all timeout values but. as we know, only read is implemented so far). Now, does disabling mean non-blocking or infinite blocking? Implemented is the former, @johnson325 once wrote a change (siemens@a1bee1d#diff-e8bb0dd62dcd38efa1cafbb258e5b0a4a46624f361b3ec05329a4e10d08bccf0L675) for our patch queue that assumed the latter. Trying to sort this out right now.