Introduce intermediate build target for std.

This gives a place for common functions to be defined.

Change-Id: Ifac38e647ab1042a00d224a58e28a0e6807fa60c
diff --git a/src/fdt.c b/src/fdt.c
index 3859b46..b781a72 100644
--- a/src/fdt.c
+++ b/src/fdt.c
@@ -18,9 +18,8 @@
 
 #include <stdint.h>
 
-#include "hf/arch/std.h"
-
 #include "hf/dlog.h"
+#include "hf/std.h"
 
 struct fdt_header {
 	uint32_t magic;