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/Makefile

@@ -5,15 +5,22 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
+# Use upstream code.
 obj-y += \
 	fdt.o \
-	fdt_ro.o \
-	fdt_rw.o \
 	fdt_strerror.o \
 	fdt_sw.o \
-	fdt_wip.o \
 	fdt_empty_tree.o \
-	fdt_addresses.o \
-	fdt_region.o
+	fdt_addresses.o
 
 obj-$(CONFIG_OF_LIBFDT_OVERLAY) += fdt_overlay.o
+
+# Locally modified for U-Boot.
+# TODO: split out the local modifiction.
+obj-y += \
+	fdt_ro.o \
+	fdt_rw.o \
+	fdt_wip.o \
+
+# U-Boot own file
+obj-y += fdt_region.o