libfdt: move working_fdt and FDT_RAMDISK_OVERHEAD to include/libfdt.h

libfdt_env.h exists to contain system-dependent defines:

  - typedef of fdt*_t
  - fdt*_to_cpu(), cpu_to_fdt*

working_fdt and FDT_RAMDISK_OVERHEAD are unrelated to the environment,
so they must get out of this header.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>