…d rollback protection.

Remove last of the saturated math calls.
Rename FlashRange::{store,exec} to {stored,at_runtime} for clarity.
Remove unnecessary image validation logic in fn padded_image_len.

Don't require updating to hubtools 0.4.7. There doesn't need to be
an 'EPOC' in the caboose as it is always "0" at this step in
the release engineering flow.

Fix complementary swapped args and corresponding swapped comparison to
read correctly.

Remove vestigial epoch header check. With the decision to not use
ImageHeader.epoch, a TBD prep_image_update variant will contain
the epoch information.