AP_HAL_ESP32: allow UART0 to use USB Serial/JTAG

[muramura]

Summary

Allow ESP32 boards to opt in to using ESP32-S3 USB Serial/JTAG for UART0.

This enables M5StampFly to use USB Serial/JTAG as the console/MAVLink port instead of the unused UART0 GPIO pins.

Classification & Testing

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

Built Copter for esp32s3m5stampfly.

Flashed to M5StampFly and confirmed bidirectional MAVLink communication over /dev/ttyACM0 using ESP32-S3 USB Serial/JTAG:

mavproxy.py --master=/dev/ttyACM0 --baudrate 921600

Confirmed MAVLink heartbeat, parameter download, and command input.

Description

M5StampFly is an ESP32-S3 based vehicle. Its UART0 GPIO pins are not useful as an external telemetry connection, but ESP32-S3 provides USB Serial/JTAG over the USB connector.

This PR adds an opt-in HAL_ESP32_UART0_USB_SERIAL_JTAG hwdef define. Existing ESP32 board behavior is unchanged unless a board explicitly enables this define.

When enabled for UART0, the ESP32 UART driver uses the ESP-IDF USB Serial/JTAG driver for reads and writes. This allows the console/MAVLink port to work over USB while keeping SERIAL0_PROTOCOL as the normal MAVLink protocol selection. SERIAL0_BAUD remains a compatibility setting; the physical transport is USB, not a GPIO UART baudrate.