Add flash script and documentation for XIAO ESP32-C6

[gonzague]

Summary

Adds an automated flash script and comprehensive documentation for flashing firmware to XIAO ESP32-C6 devices.

New Files

• flash.sh - Automated flashing script
• HOWTO-FLASH.md - Step-by-step flashing guide

Flash Script Features

• Auto-detects USB serial port
• Prompts for selection when multiple devices connected
• Verifies ESP32-C6 chip type during upload (rejects wrong chips)
• Displays device MAC address for identification
• Supports all build environments (debug, release, debugSSDP, releaseSSDP)
• Options:
  • -e, --env - Select build environment
  • -s, --skip-build - Use existing firmware (faster for multiple devices)
  • -f, --skip-fs - Skip filesystem upload
  • -y, --yes - Auto-confirm prompts
  • -m, --monitor - Open serial monitor after flash

Usage Examples

# Standard flash (debug build)
./flash.sh

# Flash multiple devices quickly
./flash.sh -e release -s -y

# Flash and monitor
./flash.sh -m

Documentation Includes

• Hardware requirements and CC1101 wiring table
• PlatformIO installation instructions
• Step-by-step flashing guide
• Partition layout explanation
• Troubleshooting section
• Command reference