binman: Allow overriding BuildSectionData()

This method is currently marked private. However it is useful to be able
to subclass it, since much of the entry_Section code can be reused. Rename
it.

Also document one confusing part of this code, so people can understand
how to add a test for this case.

Fix up a few pylint warnings to avoid regressing the score.

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