tee: Rework Makefile logic

tee: Rework Makefile logic

The intention of how this Makefile was written was to allow for sandbox
to build and test drivers still while otherwise requiring OPTEE to be
enabled. This however didn't work quite right in practice as sandbox
could enable some drivers which would then fail to link. Rework things
such that sandbox will also traverse the optee directory when
SANDBOX_TEE is enabled, but only build one of the optee-specific files
when OPTEE is enabled.

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>