binman: Move section-building code into a function

Create a new _BuildSectionData() to hold the code that is now in
GetData(), so that it is clearly separated from entry.GetData() base
function.

Separate out the 'pad-before' processing to make this easier to
understand.

Unfortunately this breaks the testDual test. Rather than squash several
patches into an un-reviewable glob, disable the test for now.

This also affects testSkipAtStartSectionPad(), although it still not
quite what it should be. Update that temporarily for now.

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