When the DEBUG config was set, the optee_os was still built with
DEBUG=0 because the flag was hardcoded. As a result the optimization
was still O2. This is fixed by using the global DEBUG flag instead of
the hardcoded value of zero.

Signed-off-by: Hashem Tatari <hashem.tatari@exset.com>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>