zephyr/Kconfig: Add downgrade prevention to swaps

Downgrade prevention for swap upgrades that was added to
mcuboot is now configurable in zephyr.

It may be using software version number from image in slot 0,
or security counter from the image in slot 0 (for limited downgrade
availability).

Hardware base security counter check remains unchanged.

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>