zephyr: Enable MPU flash writing
zephyr: Enable MPU flash writing

Recent Zephyr trees require this config option to be set in order to be
able to write to the flash device.  Without it, targets with an MPU
enabled will abort during startup, usually before even printing a
message to the UART.

Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>

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