zephyr-poc: Produce working end-to-end demo

[kurtjd]

Once support for all the services has been added to the PoC, validate functionality using the ec-test-tui app. Ensure the tui app can talk to the Zephyr PoC over serial from host connected to dev board and correctly send and receive data from the services.

Depends on #869 #870 #871 #872