net: sntp: Implement SNTP client library

The implementation is based on net app API. It sends the request and
parses the server reply by following some suggestions mentioned in the
secion "SNTP Server Operations" of RFC 4330.

The system uptime is used as the transmit timestamp of client request
This lib can work on those devices without RTC.

Signed-off-by: Aska Wu <aska.wu@linaro.org>