Introduce intermediate build target for std.

This gives a place for common functions to be defined.

Change-Id: Ifac38e647ab1042a00d224a58e28a0e6807fa60c
diff --git a/src/fdt_handler.c b/src/fdt_handler.c
index 97cdd2b..0a0b294 100644
--- a/src/fdt_handler.c
+++ b/src/fdt_handler.c
@@ -16,14 +16,13 @@
 
 #include "hf/fdt_handler.h"
 
-#include "hf/arch/std.h"
-
 #include "hf/boot_params.h"
 #include "hf/cpu.h"
 #include "hf/dlog.h"
 #include "hf/fdt.h"
 #include "hf/layout.h"
 #include "hf/mm.h"
+#include "hf/std.h"
 
 static uint64_t convert_number(const char *data, uint32_t size)
 {