binman: Allow state functions to fail to return data

binman: Allow state functions to fail to return data

At present these state functions raise an exception if they cannot find
what is requested. But in some cases the information is optional (e.g. an
fdtmap in a coming patch) so it is better to return gracefully.

Update these two functions to return None when the data cannot be found.

Signed-off-by: Simon Glass <sjg@chromium.org>