boot: zephyr: ensure LOG_LEVEL resource savings
boot: zephyr: ensure LOG_LEVEL resource savings

Let's ensure that random Zephyr changes in logging don't affect
the resource usage in MCUBOOT, by setting LOG_LEVEL_DEFAULT
to 0.

Signed-off-by: Michael Scott <a17fed27eaa842282862ff7c1b9c8395a26ac320@foundries.io>
Signed-off-by: Andrzej Puzdrowski <0222c24e1773dd88269ed0e559909ec9bea3604a@nordicsemi.no>

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