[TF-M] Changes to TF-M NS interface build

Hi all,

Happy holiday!

I'd like to let you know that non-secure interface build has been moved from TF-M to tf-m-tests in following patches: TF-M: https://review.trustedfirmware.org/q/topic:%22move-ns-api%22+(status:open%20...) Tf-m-tests: https://review.trustedfirmware.org/q/topic:%22add-ns-api%22+(status:open%20O...)

After that change, the .c files under TF-M interface/src are built during non-secure side build. It aligns with the actual process of NS integration with TF-M. Moreover, non-secure specific flags, which control TF-M NS interface, can be configured and managed in non-secure side build, without affecting TF-M secure part.

Please modify tf-m-tests app/CMakeLists.txt for future changes to non-secure interface build. Thanks.

Best regards, Hu Ziji