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>
diff --git a/cmake/install.cmake b/cmake/install.cmake
index 981bfba..106369e 100644
--- a/cmake/install.cmake
+++ b/cmake/install.cmake
@@ -56,6 +56,7 @@
     install(FILES       ${INTERFACE_INC_DIR}/multi_core/tfm_multi_core_api.h
                         ${INTERFACE_INC_DIR}/multi_core/tfm_ns_mailbox.h
                         ${INTERFACE_INC_DIR}/multi_core/tfm_mailbox.h
+                        ${INTERFACE_INC_DIR}/multi_core/tfm_mailbox_config.h
             DESTINATION ${INSTALL_INTERFACE_INC_DIR})
 else()
     install(FILES       ${CMAKE_BINARY_DIR}/generated/interface/include/tfm_veneers.h