lib: libfdt: wrap scripts/dtc/libfdt/* where possible

lib/libfdt/ and scripts/dtc/libfdt have the same copies for the
followings 6 files:
  fdt.c fdt_addresses.c fdt_empty_tree.c fdt_overlay.c fdt_strerr.c
  fdt_sw.c

Make them a wrapper of scripts/dtc/libfdt/*.  This is exactly what
Linux does to sync libfdt.  In order to make is possible, import
<linux/libfdt.h> and <linux/libfdt_env.h> from Linux 4.14-rc5.

Unfortunately, U-Boot locally modified the following 3 files:
  fdt_ro.c fdt_wip.c fdt_rw.c

The fdt_region.c is U-Boot own file.

I did not touch them in order to avoid unpredictable impact.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

lib/libfdt/fdt_empty_tree.c

@@ -1,37 +1,2 @@
-/*
- * libfdt - Flat Device Tree manipulation
- * Copyright (C) 2012 David Gibson, IBM Corporation.
- * SPDX-License-Identifier:	GPL-2.0+ BSD-2-Clause
- */
-#include <libfdt_env.h>
-#include <fdt.h>
-#include <libfdt.h>
-
-#include "libfdt_internal.h"
-
-int fdt_create_empty_tree(void *buf, int bufsize)
-{
-	int err;
-
-	err = fdt_create(buf, bufsize);
-	if (err)
-		return err;
-
-	err = fdt_finish_reservemap(buf);
-	if (err)
-		return err;
-
-	err = fdt_begin_node(buf, "");
-	if (err)
-		return err;
-
-	err =  fdt_end_node(buf);
-	if (err)
-		return err;
-
-	err = fdt_finish(buf);
-	if (err)
-		return err;
-
-	return fdt_open_into(buf, buf, bufsize);
-}
+#include <linux/libfdt_env.h>
+#include "../../scripts/dtc/libfdt/fdt_empty_tree.c"