Restrict execution of TEE client applications to members of the teeclnt
group. A test user is created and added to this group.
Having two groups (tee and teeclnt) allows to better isolate
tee-supplicant from the client applications.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>

Adds external packages repository for buildroot containing:
* Packing for optee_client, optee_examples, optee_test and optee_benchmark
* Configs and a script to assembly defconfigs needed for buildroot

Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>