App: Create a dedicated mailbox thread

Create a dedicated mailbox thread to align with TF-M dual-cpu NS
mailbox new working model.
Update other NS mailbox APIs to align with TF-M changes.

Signed-off-by: David Hu <david.hu@arm.com>
Change-Id: I1eac4b84085b9e5a329cf82f7bf4e017fcf6062a
3 files changed