Dualcpu: Move NS mailbox thread management to RTOS specific file

The NS mailbox thread management is NS RTOS specific.

- Add a new tfm_ns_mailbox_rtos_api.c to collect all the NS mailbox
  APIs which rely on RTOS specific implementations.
- Move those API implementations from platform to
  tfm_ns_mailbox_rtos_api.c.
- Rename those APIs to replace HAL keyword with OS keyword, to
  indicate that their implementations are RTOS specific.

Change-Id: Ic2885bc1676964719d1524b39d6518444610e1aa
Signed-off-by: David Hu <david.hu@arm.com>
8 files changed