binman: Allow use of help and entry-docs without libfdt

binman: Allow use of help and entry-docs without libfdt

At present if libfdt is not available binman can't do anything much.
Improve the situation a little.

Ideally there should be a test to cover this, but I'm not quite sure how
to fake this.

Signed-off-by: Simon Glass <sjg@chromium.org>
(fixed up missing ReadChildData() enty test)