libfdt: migrate fdt_rw.c to a wrapper of scripts/dtc/libfdt/fdt_rw.c

The only difference between scripts/dtc/libfdt/fdt_rw.c and
lib/libfdt/fdt_rw.c is fdt_remove_unused_strings().

It is only used by fdtgrep, so we do not need to compile it for U-Boot
image.  Move it to tools/libfdt/fdw_rw.c so that lib/libfdt/fdt_rw.c
can be a wrapper of scripts/dtc/libfdt/fdt_rw.c.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/libfdt/Makefile b/lib/libfdt/Makefile
index be42e94..4e3e12f 100644
--- a/lib/libfdt/Makefile
+++ b/lib/libfdt/Makefile
@@ -11,6 +11,7 @@
 	fdt_wip.o \
 	fdt_strerror.o \
 	fdt_sw.o \
+	fdt_rw.o \
 	fdt_empty_tree.o \
 	fdt_addresses.o
 
@@ -18,9 +19,7 @@
 
 # Locally modified for U-Boot.
 # TODO: split out the local modifiction.
-obj-y += \
-	fdt_ro.o \
-	fdt_rw.o
+obj-y += fdt_ro.o
 
 # U-Boot own file
 obj-y += fdt_region.o