br-ext: install OP-TEE devkit TAs

Install all TAs found in devkit into target filesystem. This change
allows OP-TEE test environment to exercise in-tree TAs as the pkcs11 TA.

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