Adds notes on how to run the two main test suites for TF-M using
the supplied sample applications.
Signed-off-by: Kevin Townsend <kevin.townsend@linaro.org>
This commit update the NS board variant from `mps2_an521_nonsecure`
to `mps2_an521_ns` to maintain consistency across zephyr.
Signed-off-by: Kevin Townsend <kevin.townsend@linaro.org>