buildroot: add teeclnt group and test user account

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>