Implement device firmware upgrade using MCUboot Serial Recovery over
usart1 (reusing the existing protocol UART). The application layer
handles upgrade commands (0xFF) to trigger bootloader entry, while
MCUboot manages the actual firmware swap with RSA signature
verification.
Key additions:
- sysbuild.conf to enable MCUboot bootloader with signing key
- mcuboot.conf with serial recovery and watchdog feed settings
- Kconfig option APP_DFU (default y) with conditional compilation
- dfu.cpp module for DFU initialization
- Upgrade command callback in com.hpp (bootmode_set + sys_reboot)
- Documentation for upgrade procedures and change notes
