zephyr: fix build for nrf52840_pca10059
zephyr: fix build for nrf52840_pca10059

The build overflows by 100B using the 7-2018-q2-update GNU Arm
Embedded toolchain. Turn on size optimizations; this leaves about 19%
of the partition's flash available for MCUboot to grow into.

Signed-off-by: Marti Bolivar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@nordicsemi.no>

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