binman: Adjust fmap to ignore CBFS files

The FMAP is not intended to show the files inside a CBFS. The FMAP can be
used to locate the CBFS itself, but then the CBFS must be read to find out
what is in it.

Update the FMAP to work this way and add some debugging while we are here.

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