Enable USB-based firmware flashing for MKS Robin Nano v3.1 board

[andre-stefanov]

The MKS Robin Nano v3.1 board currently uses a proprietary bootloader which only allows firmware updates via SD card. For this firmware, it is necessary to support firmware flashing via USB.

Potential solutions such as mcuboot or tinyuf2 should be considered and evaluated for integration, allowing users to update firmware through USB rather than relying solely on SD card flashing.

Acceptance Criteria

• Evaluate mcuboot and tinyuf2 bootloader options for MKS Robin Nano v3.1
• Select and implement the best USB bootloader solution
• USB DFU flashing works on MKS Robin Nano v3.1 hardware
• Update build system to generate compatible firmware
• Update documentation with USB flashing instructions
• Test firmware flashing and operation via USB