mk/aosp_optee.mk: Allow extra flags for aosp builds
mk/aosp_optee.mk: Allow extra flags for aosp builds

Signed-off-by: Victor Chong <4aabb03bd6dd29c44a49684d87b08ac07e10b492@linaro.org>
Reviewed-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>
[jf: rebase on top of master]
Signed-off-by: Jerome Forissier <ced78d268b4c9a0a20a59a9feeebd2ad90da03d3@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os