binman: Show the image name for the top-level section

At present we show 'main section' as the top-level section name. It may
be more helpful to show the actual image name. This is tricky because
Image is a parent class of Entry_section, so there is no distinction
between an image and a section.

Update it to show the image name.

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