binman: Avoid calculated section data repeatedly

Refactor the implementation slightly so that section data is not
rebuilt when it is already available.

We still have GetData() set up to rebuild the section, since we don't
currently track when things change that might affect a section. For
example, if a blob is updated within a section, we must rebuild it.
Tracking that would be possible but is more complex, so it left for
another time.

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