Zigbee ota updates

[oleo65]

This is a first attempt at activating the zigbee ota functionality. All changes are toogled via Kconfig parameter.

Vastly inspired by the zigbee light switch sample from the nrf-toolkit.

This works so far in adding an additional OTA cluster to the device and publishing the flashed firmware version.

I was not able to get a firmware update on the device, but this is most probably due to the quite complicated process of providing the updates to the zigbee controller.

This addresses #128