Add Zephyr support for home-automation demo

[task-jp]

• Add home-automation demo with Zephyr integration
• Consolidate shared Zephyr configuration into demos/zephyr-common/
  • boards/ configs, prj.conf, west.yaml, slint-zephyr platform layer
• Update printerdemo to use shared configuration

[tronical]

Sounds like a good idea to me to collect common files in a common folder, as a starting point.

cc @0x6e