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/psa_client.h b/interface/include/psa_client.h
index dbe0e72..1fc8637 100644
--- a/interface/include/psa_client.h
+++ b/interface/include/psa_client.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, Arm Limited. All rights reserved.
+ * Copyright (c) 2018-2019, Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  *
@@ -32,6 +32,7 @@
 
 typedef int32_t psa_error_t;
 typedef int32_t psa_handle_t;
+typedef int32_t psa_status_t;
 
 /**
  * A read-only input memory region provided to a RoT Service.