OP-TEE OS supports custom tarballs. If the OP-TEE OS custom tarball version does not match the latest optee-test version supported by Buildroot, optee-test might not build or run properly. This patch adds support for an optee-test custom tarball URL to address this potential issue. Signed-off-by: Kory Maincent <kory.maincent@bootlin.com> Acked-by: Etienne Carriere <etienne.carriere@foss.st.com> Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>