binman: Set section contents in GetData()

Section contents is not set up when ObtainContents() is called, since
packing often changes the layout of the contents. Ensure that the contents
are correctly recorded by making this function regenerate the section. It
is normally only called by the parent section (when packing) or by the
top-level image code, when writing out the image. So the performance
impact is fairly small.

Now that sections have their contents in their 'data' property, update
testSkipAtStartSectionPad() to check it.

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