esp32/machine_uart: Implement UART.sendbreak() method.
esp32/machine_uart: Implement UART.sendbreak() method.

The uart_write_bytes_with_break() function requires non-zero data to be
sent before the break, so a standalone break must be synthesised.

kerneltask/micropython,adafruit/circuitpython,selste/micropython,adafruit/circuitpython,adafruit/circuitpython,pramasoul/micropython,henriknelson/micropython,pramasoul/micropython,tobbad/micropython,bvernoux/micropython,tobbad/micropython,adafruit/circuitpython,MrSurly/micropython,MrSurly/micropython,MrSurly/micropython,pozetroninc/micropython,pramasoul/micropython,trezor/micropython,kerneltask/micropython,adafruit/circuitpython,pramasoul/micropython,pfalcon/micropython,pfalcon/micropython,henriknelson/micropython,selste/micropython,selste/micropython,MrSurly/micropython,henriknelson/micropython,bvernoux/micropython,kerneltask/micropython,kerneltask/micropython,tobbad/micropython,adafruit/circuitpython,trezor/micropython,pfalcon/micropython,henriknelson/micropython,pozetroninc/micropython,selste/micropython,pfalcon/micropython,trezor/micropython,pfalcon/micropython,trezor/micropython,bvernoux/micropython,selste/micropython,tobbad/micropython,pramasoul/micropython,bvernoux/micropython,kerneltask/micropython,trezor/micropython,tobbad/micropython,pozetroninc/micropython,pozetroninc/micropython,pozetroninc/micropython,henriknelson/micropython,MrSurly/micropython,bvernoux/micropython