binman: Update Entry.WriteData() to handle special sections

At present this method assumes that the parent section does not need
to recalculate its position or adjust any metadata it may contain. But
when the entry changes size this may not be true. Also if the parent
section is more than just a container (e.g. it is a CBFS) then the
section may need to regenerate its output.

Add a new WriteChildData() method to sections and call this from the
WriteData() method, to handle this situation.

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