zephyr: boot: Correct how the default board is set
zephyr: boot: Correct how the default board is set

The default board should be set simply by 'set'ing the BOARD. This has
the lowest precedence of all possible inputs.

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

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