Fix overwrite-only under Zephyr
Fix overwrite-only under Zephyr

As reported by issue #384, some #ifdefery was wrongly done, which broke
overwrite-only mode under Zephyr.

Signed-off-by: Fabio Utzig <66676b1ceaf93296e098708c09494361103aa635@apache.org>

utzig/mcuboot,runtimeco/mcuboot,utzig/mcuboot,utzig/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,utzig/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,tamban01/mcuboot