azure: Retry NTP request in case of failure

Sometimes, pool.ntp.org does not reply to requests. When this
happens, the ntp client blocks forever.

To solve that issue, a timeout of 3s is set on the ntp socket. The
timeout in the azure stack has been raised to 15s to accomodate this
change.

Finaly since the azure example require custom lwip options; it is now
isolated from examples that uses the default lwip configuration.

Signed-off-by: Vincent Coubard <vincent.coubard@arm.com>