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>

tools/binman/ftest.py

@@ -4533,7 +4533,7 @@ class TestFunctional(unittest.TestCase):
     def testCollectionSection(self):
         """Test a collection where a section must be built first"""
         # Sections never have their contents when GetData() is called, but when
-        # _BuildSectionData() is called with required=True, a section will force
+        # BuildSectionData() is called with required=True, a section will force
         # building the contents, producing an error is anything is still
         # missing.
         data = self._DoReadFile('199_collection_section.dts')