azure: Terminate NTP task explicitely

When a thread returns from its starting function; it does not
consistently terminate the threads accross CMSIS RTOS implementations.
On RTX, the thread exits properly while it crashes the system on
FreeRTOS.

This change fixes this behavior in the NTP task of the azure client. The
ntp thread is now explicitely terminated when stopped.

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