This task aims to bring better and easier RTOS integration by making the following changes to the NS interfaces exported to build/install/export/tfm:

1. Split source files of NS programming API for different models E.g. Will split tfm_sst_api.c to tfm_sst_func_api.c for library model and tfm_sst_ipc_api.c for IPC model, each will only contains the implementation for that model.
2. Export the interface files for different build configs on necessity E.g. tfm_sst_func_api.c will not be exported for builds of IPC model
3. Refine of the tfm_ns_interface_dispatch function will be covered by T378