west: try image-1's size as slot size in imgtool
west: try image-1's size as slot size in imgtool

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 <382671175562e0df8128054dbdb05a250d79f588@nordicsemi.no>

galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr