boot: bootutil: Add swap using offset algorithm

Adds a new variation of the swap using move mode, named swap using
offset, whereby instead of moving all the sectors in the primary
image, the sectors in the secondary image are offset instead. This
fastens image swapping time both for updates and reverts as each
sector in both slots is erased only once, which also reduces flash
wear, and uses less swap status bits to represent

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>