boot: zephyr: Clean up before chainloading by default

This changes the default mcuboot configuration for zephyr
applications to clean up before booting the user application. The
reason for this change is that mcuboot may configure protection (e.g.
MPU stack guard) which is then used by the user application during
its startup code prior to cleaning the configuration up, this can
lead to a unbootable application and potentially irrecoverable
module, therefore cleaning up is now being enabled by default.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>