tools: use files from scripts/dtc/libfdt where possible

Prior to this commit, tools/Makefile pulls all libfdt files from
lib/libfdt.

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

This commit changes them to #include ones from scripts/dtc/libfdt.

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

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

The fdt_region.c is U-Boot own file.  This is also borrowed from
lib/libfdt/.

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