zephyr: Enable interrupts unconditionally
zephyr: Enable interrupts unconditionally

Due to an issue described here:
https://github.com/zephyrproject-rtos/zephyr/issues/8393
interrupts are not enabled when multithreading is disabled.
Enable interrupts to allow the serial recovery mode UART to receive
characters.

Note: This commit must be reverted once the issue is addressed.

Signed-off-by: Carles Cufi <853081dc8dca4fe6a7e3c984752718b5db36e060@nordicsemi.no>

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