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/interface/include/tfm_api.h b/interface/include/tfm_api.h
index 8dbf57c..be36e2e 100644
--- a/interface/include/tfm_api.h
+++ b/interface/include/tfm_api.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2018, Arm Limited. All rights reserved.
+ * Copyright (c) 2017-2019, Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  *
@@ -41,11 +41,13 @@
 
 /* FixMe: sort out DEBUG compile option and limit return value options
  * on external interfaces */
-/* Note:
+/* For secure functions using prorietary signatures
  * TFM will only return values recognized and parsed by TFM core.
  * Service return codes are not automatically passed on to REE.
  * Any non-zero return value is interpreted as an error that may trigger
  * TEE error handling flow.
+ * For secure functions using the veneers in secure_fw/ns_callable/tfm_veneers.c
+ * (iovec API) this limitation does not apply.
  */
 enum tfm_status_e
 {