samples: Add mpu stack guard test

This patch adds the mpu_stack_guard_test to the available samples.
The purpose of this test is to exercize the thread stack guard enabled
via MPU on arm platforms.

Change-Id: I4665a20956d9e6d0dd4b5cc862e82040a53afafc
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>