binman: Add a function to obtain the image for an Entry

binman: Add a function to obtain the image for an Entry

At present we have an 'image' property in the entry for this purpose, but
this is not necessary and seems error-prone in the presence of
inheritance. Add a function instead. The Entry_section class overrides
this with a special version, since top-level sections are in fact images,
since Image inherits Entry_section.

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