Add buildroot external packages repository

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>