Introduce intermediate build target for std.

This gives a place for common functions to be defined.

Change-Id: Ifac38e647ab1042a00d224a58e28a0e6807fa60c
diff --git a/src/cpio.c b/src/cpio.c
index 6f2442d..61a82d6 100644
--- a/src/cpio.c
+++ b/src/cpio.c
@@ -18,7 +18,7 @@
 
 #include <stdint.h>
 
-#include "hf/arch/std.h"
+#include "hf/std.h"
 
 #pragma pack(push, 1)
 struct cpio_header {