Signed-off-by: NWolfgang Denk <wd@denx.de>

Enhanced the formerly private function _fdt_next_tag() to allow stepping
  through the tree, used to produce a human-readable dump, and made
  it part of the published interface.
Also added some comments.

This adds the applicable libfdt source files (unmodified) and a README
to explain where the source came from.