zephyr: add cleanup ARM core before boot

This patch is needed as MCUBoot should be able to chain-load any
application, not only these built using zephyr.

Introduced cleanup on ARM core control register.
Might be required as for instance the application assumes
that it starts with thread mode configured as by default, not
according to zephyr-rtos configuration.

MCUBoot disables interrupt before application chain-load used
basepr register. This Patch introduce additional celenup on
NVIC register.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>