Add downgrade prevention for swaps

Currently, downgrade prevention was limited to overwrite only
builds (version check) or devices with hardware storage for
security counter.

This extends downgrade prevention to be used when swap update
is selected.
Unlike MCUBOOT_HW_ROLLBACK_PROT option it does not require user
code to provide external way to store security counter.
Security counter from slot 1 image is used for comparison.
With security counter usage it is possible to have limited
software rollback if security counter was not incremented.

It is possible to use image version where strict rule for
image version comparison prevents any downgrades.

Downgrade prevention is also added to mynewt configuration.

If image in slot 1 is marked as pending and downgrade prevention
is in place, image will be deleted to avoid check on next boot.

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