boot: bootutil: swap_scratch: Fix issue with bricking device

Fixes an issue with the swap using scratch algorithm that would
cause the image loaded into the primary slot to be corrupt and
unbootable if a device was rebooted during an erase of the scratch
section that had not completed

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
(cherry picked from commit f76fba70)