CircuitPython on Zephyr

[tannewt]

We'd like to use Zephyr as the common real-time operating system (RTOS) across all of CircuitPython. This will allow us to:

• Utilize an RTOS for sharing computation like synthio and displayio across multiple cores.
• Unify Bluetooth, WiFi, Thread and Mobile APIs onto one underlying API.
• Unify flash and time management.
• Unify build system.
• Minimally support every Zephyr supported platform. (AKA Reduce the engineering cost of supporting new platforms.)