Build: Improve the setting of number of mailbox queue slots

Move configuration of number of mailbox queue slots from platform's
device_cfg.h into a common header file tfm_mailbox_config.h.

tfm_mailbox_config.h is automatically generated during build. The
value of NUM_MAILBOX_QUEUE_SLOT is passed from build configuration
and then set in tfm_mailbox_config.h.

Change-Id: I9a3ac465b71b316accf6cd41cea80745eec8607c
Signed-off-by: David Hu <david.hu@arm.com>
8 files changed