This cannot be summarized shortly – one has to do board definition changes to add zephyr in the list of supported frameworks, then platform-espressif8266 needs to get the zephyr.py builder script added to it, the main.py script needs be adapter, the platform.json needs to be expanded to list all the framework-zephyr-* packages, etc… Very technical internal PlatformIO development stuff. Best to just let PIO developers build the support for it.
Wait a second there. When I look in the Zephyr documentation it talks about using an ESP8266 as a shield / module to give another base board a WiFi extension. Not compiling a Zephyr firmware that actually runs on the ESP8266 chip.