samples: sockets: sntp_client: make IPv6 & NET_CONFIG_SETTINGS optional

In this commit, a config.h header file is added to support the case
when the NET_CONFIG_SETTINGS is not used. main.c is also modified to
enable platforms that do not support IPv6. These changes are necessary
in order to allow cc3220sf_launchxl support.

Signed-off-by: Vincent Wan <vincent.wan@linaro.org>