Add support for gp-suite in optee_test

By passing GP_PACKAGE=<path to GP-suite archive> optee_test is
configured to compile with the GlobalPlatform test suite. The test suite
is unpacked and patched in the build directory as part of the buildroot
process when building optee_test.

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