Conditionally enable Thread Local Storage tests in the
optee_test_ext Buildroot package. Some toolchains may not support this
feature [1] so it is best to let the user decide.

common.mk enables the tests by default since the GCC toolchain from
toolchain.mk supports them. The command line option is
'make WITH_TLS_TESTS=y|n'.

Link: https://github.com/OP-TEE/optee_test/commit/30efcbeaf8864d0f2a5c4be593a5411001fab31b


Signed-off-by: Jerome Forissier <jerome@forissier.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>