New OVERWRITE_ONLY_KEEP_BACKUP option

It builds on top of OVERWRITE_ONLY mode and uses secondary slot
as a backup of the primary slot. The main difference is that after
image copy to the primary slot the secondary slot is not erased.
This is meant to be used together with BOOTSTRAP option that will
reinstall the primary image with the backup in case it's not valid.

Signed-off-by: Petr Buchta <petr.buchta@nxp.com>