tee: sandbox: check for buffer size

Add additional check for buffer size when reading out persistent
storage value and provide back actual value size.

Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Reviewed-by: Oleksandr Suvorov <cryosay@gmail.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>