Add negative testing to message_handler

Also restructure the scatter file to not be constrained of the
artificial 512k size limit of the APP_IMAGE region.

Add missing sections in DDR for both scatter file and linker script.

Change-Id: I3d9bc8aeae1b1c11ab994276be64a2850cc23f8e
diff --git a/applications/message_handler/test/message_client.cpp b/applications/message_handler/test/message_client.cpp
index 4209564..39d1392 100644
--- a/applications/message_handler/test/message_client.cpp
+++ b/applications/message_handler/test/message_client.cpp
@@ -34,7 +34,7 @@
 
 bool MessageClient::sendInputMessage(const uint32_t type, const void *src, uint32_t length) {
     if (!input.write(type, src, length)) {
-        printf("ERROR: Msg: Failed to write ping request. No mailbox message sent\n");
+        printf("ERROR: Msg: Failed to write message request. No mailbox message sent\n");
         return false;
     }
 
@@ -44,7 +44,7 @@
 }
 
 bool MessageClient::waitAndReadOutputMessage(const uint32_t expected_type, uint8_t *dst, uint32_t length) {
-    constexpr TickType_t delay    = pdMS_TO_TICKS(2);
+    constexpr TickType_t delay    = pdMS_TO_TICKS(5);
     constexpr TickType_t deadline = pdMS_TO_TICKS(/* 1 minute */ 60 * 1000 * 1000);
     struct ethosu_core_msg msg;
 
@@ -68,7 +68,7 @@
     }
 
     if (msg.type != expected_type) {
-        printf("ERROR: Wrong message type\n");
+        printf("ERROR: Wrong message type. Got %" PRIu32 " expected %" PRIu32 "\n", msg.type, expected_type);
         return false;
     }