Introduce intermediate build target for std.

This gives a place for common functions to be defined.

Change-Id: Ifac38e647ab1042a00d224a58e28a0e6807fa60c
diff --git a/test/hftest/inc/hftest_impl.h b/test/hftest/inc/hftest_impl.h
index 328f664..a86069c 100644
--- a/test/hftest/inc/hftest_impl.h
+++ b/test/hftest/inc/hftest_impl.h
@@ -18,9 +18,8 @@
 
 #include <stdnoreturn.h>
 
-#include "hf/arch/std.h"
-
 #include "hf/spci.h"
+#include "hf/std.h"
 
 #define HFTEST_MAX_TESTS 50