espressif UART: clear input buf on creation to avoid stray byte

[dhalbert]

• Fixes ESP32-S3: Stray byte read on UART when first created #8890.

On some Espressif chips (noticed on ESP32-S3 and ESP32-C3, not ESP32-S2, did not test others), a strayjunk byte of some number of consecutive 1's may appear as the first byte in the input buffer. Easy fix is to wait at least one byte time and then clear the input buffer.

Test program:

import busio
import board

for baudrate in (110, 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200, 230400, 468000, 921600):
    u = busio.UART(board.TX, board.RX, baudrate=baudrate)
    print(baudrate, u.read())
    u.deinit()

When fixed, should print all Nones.

[tannewt]

Thank you!