Establish Zephyr port

[tannewt]

Establish a baseline ports/zephyr that works for every Zephyr board.

Include:

• Fixed implementations of I2C, UART and SPI that are linked into board.
• Flexible digitalio that uses the Zephyr API.
• Default CIRCUITPY storage.
• west build support that uses async Python to build the CircuitPython bits.

[tannewt]

The fixed implementation versions of I2C, UART and SPI will live in modules separate from busio but act the same way. busio is reserved for dynamic pin construction.