Add hardware flow control pin definitions

[puddly]

If hardware flow control is enabled by setting the RTS and CTS pins to a positive number, we switch the UART to UART_HW_FLOWCTRL_CTS_RTS. This is disabled by default.

[Copilot]

Pull Request Overview

This PR adds hardware flow control support to the USB UART bridge by introducing RTS and CTS pin definitions. Hardware flow control is conditionally enabled based on the pin configuration values.

• Added configuration options for UART RTS and CTS pins with default values of -1 (disabled)
• Implemented conditional hardware flow control enabling when both RTS and CTS pins are set to positive values
• Updated UART pin configuration to include the new RTS and CTS pins

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
main/usb_uart_bridge_main.c	Added pin definitions and conditional flow control logic with updated UART configuration
main/Kconfig.projbuild	Added configuration options for RTS and CTS pins with default disabled values

[Copilot]

The condition only checks if either pin is negative, but hardware flow control requires both RTS and CTS pins to be valid. The condition should use && instead of || to ensure both pins are non-negative before enabling flow control.

Suggested change

	#if BOARD_UART_RTS_PIN < 0 || BOARD_UART_CTS_PIN < 0
	#if BOARD_UART_RTS_PIN < 0 && BOARD_UART_CTS_PIN < 0