Core: change iovec return type to psa_status_t

 - Change the return type of the iovec veneer functions to psa_status_t.
 - Modify TF-M core to forward any return value from secure functions
   that use iovec API to the caller.
 - Update Core test services to use negative return values for
   error, and zero or positive values for successful secure function
   execution.

Change-Id: I5b42a194b955fd003c90d6dfd654ab55be483d20
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>
diff --git a/secure_fw/spm/spm_api.h b/secure_fw/spm/spm_api.h
index e1a024f..5f20e48 100644
--- a/secure_fw/spm/spm_api.h
+++ b/secure_fw/spm/spm_api.h
@@ -61,6 +61,11 @@
     uint32_t share;
     uint32_t stack_ptr;
     uint32_t lr;
+    int32_t iovec_api;               /*!< Whether the function in the partition
+                                      * had been called using the iovec API.
+                                      * FIXME: Remove the field once this is the
+                                      * only option
+                                      */
     struct iovec_args_t iovec_args;
     psa_outvec *orig_outvec;
 };