SPM: Add runtime SFN backend

A backend for implementing 'SFN'. This runtime backend is selected
if:
- All partitions included in the current build are 'SFN' partitions.
- Isolation level 1.

The configuration is generated by tooling, and output to the
generated file 'config_impl.h'.

Change-Id: I819c78c7a912f86174d102d389a2714ae17e5062
Signed-off-by: Ken Liu <Ken.Liu@arm.com>
Co-authored-by: Mingyang Sun <mingyang.sun@arm.com>
diff --git a/secure_fw/spm/include/ffm/backend.h b/secure_fw/spm/include/ffm/backend.h
index 7b7ea00..5efeac0 100644
--- a/secure_fw/spm/include/ffm/backend.h
+++ b/secure_fw/spm/include/ffm/backend.h
@@ -37,9 +37,9 @@
 
     /*
      * Runtime model-specific message replying.
-     * Return the acked status.
+     * Return the connection handle or the acked status code.
      */
-    void (*replying)(struct tfm_msg_body_t *p_msg, psa_status_t status);
+    int32_t (*replying)(struct tfm_msg_body_t *p_msg, int32_t status);
 };
 
 /* RUNTIME MODEL BACKENDS DECLARATION */