zephyr: allow to disable timers supports
zephyr: allow to disable timers supports

For some basic mcuboot configurations the zephyr timers are
not required which reduce the flash footprint significantly.

Signed-off-by: Andrzej Puzdrowski <0222c24e1773dd88269ed0e559909ec9bea3604a@nordicsemi.no>

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