binman: Allow vblock to include devicetree blobs

At present if a devicetree blob is included in a vblock it does not deal
with updates. This is because the vblock is created once at the start and
does not have a method to update itself later, after all the entry
contents are finalised.

Fix this by adjusting how the vblock is created.

Also simplify Image.ProcessEntryContents() since it effectively duplicates
the code in Section.ProcessContents().

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