Comment 2 for bug 1410370

There are 2 main reasons:

1) To test the system behaviour is consistent when given delta updates vs. full-system updates.
2) To simplify the logic when applying the very first system image update to the "other" partition (which is initially empty).
     If we force a full system update, we can:
    a) guarantee we get a fresh kernel + initrd for the "other" partition.
    b) avoid copying a -> b to then apply the downloaded delta to the "other" partition.