When MCUBoot is built using the swap-move strategy, the secondary slot
can be one sector smaller than the primary slot, because the primary
slot's last "useful" image sector must be "reserved" for the move
operation; this impacts the generation of padded images, so when
signing an image, the proper size to use is the secondary slot's,
unless a secondary image is not defined which is the case when using
single image DFU.
Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>
