samples: synch: Add Thread Stack Guards conf

This patch adds the Thread Stack Guards configuration option to the
synchronization sample.

This is meant to be an easy way to prove that the MPU can be dynamically
programmed on the supported platforms.

Change-Id: I77fbe6af3303ff8a2904768abd4cc9797f03c092
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>