Zephyr support for ESP8266

[maxgerhardt]

See https://community.platformio.org/t/add-esp8266-zephyrrtos-support/20774.

Per https://docs.zephyrproject.org/2.5.0/boards/shields/esp_8266/doc/index.html the Zephyr RTOS project supports the ESP8266.

We already have Zephyr build support for other platforms (like ststm32 and nordicnrfX), so I think it shouldn't be too difficult to add support for it too? :)

[maxgerhardt]

Sorry, I'll be revising or closing this issue after some clarifications: Zephyr doesn't run natively on the ESP8266 (but the ESP32 for example). It only supports the ESP8266 as shield, but with another MCU as a base board. I'll investigate.

https://docs.zephyrproject.org/2.5.0/boards/xtensa/index.html

[maxgerhardt]

Zephyr does not run natively on the ESP8266.