binman: Keep a separate list of entries for fit

The current implementation sets up the FIT entries but then deletes the
'generator' ones so they don't appear in the final image.

This is a bit clumsy. We cannot build the image more than once, since the
generator entries are lost during the first build. Binman requires that
calling BuildSectionData() multiple times returns a valid result each
time.

Keep a separate, private list which includes the generator nodes and use
that where needed, to correct this problem. Ensure that the missing list
includes removed generator entries too.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>