Current TFM code assumes the veneer to be the default way communication between NSPE and SPE.
The message passing mechanism needs to be abstracted from the core TFM implementation.

[Design: TBD]
Rough flow:

mbox_ISR() {

retrieve_ns_req();
post_ns_req_to_main_thread()

}

secure core main()
{

while (1) {
 wait_for_interrupt();
 dequeue_req();
 tfm_dispatch_req_to_spm();

}