Build: Explictly set TFM_PARTITION_NS_AGENT_MAILBOX flag if required

TFM_PARTITION_NS_AGENT_MAILBOX is implicitly passed to NS side from
secure side in current NSPE build implementation. This flag will not
take effect if NS and S are built independently.

Explict set this flag to NSPE if NS mailbox agent partition is enabled
in SPE to fix this issue.

Signed-off-by: David Hu <david.hu@arm.com>
Change-Id: Icbc6b49f656da7de7c134fed0a55795255fb36a9
1 file changed