Introduce intermediate build target for std.

This gives a place for common functions to be defined.

Change-Id: Ifac38e647ab1042a00d224a58e28a0e6807fa60c
diff --git a/test/vmapi/primary_with_secondaries/mailbox.c b/test/vmapi/primary_with_secondaries/mailbox.c
index 287767e..7a45dfa 100644
--- a/test/vmapi/primary_with_secondaries/mailbox.c
+++ b/test/vmapi/primary_with_secondaries/mailbox.c
@@ -16,9 +16,8 @@
 
 #include <stdint.h>
 
-#include "hf/arch/std.h"
-
 #include "hf/spci.h"
+#include "hf/std.h"
 
 #include "vmapi/hf/call.h"