zephyr: use minimal CBPRINTF implementation

Addition of cbprintf capability in zephyr
https://github.com/zephyrproject-rtos/zephyr/pull/29876


cause flash footprint rise by ~4 KB. Selecting
CBPRINTF_NANO=y mitigates the footprint rise.

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