New `optee_examples` git rep is used instead of `hello_world`, which
will contain different sample applications that will show usage of
specific TEE functionality (crypto examples, random data generation,
trusted storage etc.)

Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Igor Opaniuk <igor.opaniuk@linaro.org>