Document USB gadget networking setup for Raspberry Pi devices

[paralin]

Problem

Raspberry Pi devices (especially Pi 5, but also Pi 4, Zero, etc.) support USB gadget mode for network connectivity via the USB-C port, but this is not documented for these platforms. This would provide a static IP address for initial configuration without needing to scan the network.

Use Case

From issue #330: User suggested adding a static network interface over the USB controller (USB Type-C port) with a static IP, making initial configuration much easier without needing nmap to discover the device.

Current State

• USB gadget networking is already documented for allwinner/licheerv in configs/allwinner/licheerv/README.md
• The licheerv config provides a good template showing:
  • Host machine setup (NetworkManager and iptables methods)
  • Device-side configuration
  • IP address customization
  • Connection sharing setup

Proposed Documentation

Add similar USB gadget networking documentation to Raspberry Pi configs, specifically:

For Raspberry Pi 5, 4, Zero, etc.

1. Enable USB gadget drivers:

   • Load dwc2 kernel module
   • Configure via /etc/modules or kernel cmdline
   • Add g_ether or similar gadget module

2. Device-side configuration:

   • NetworkManager connection file for USB network
   • Default static IP (e.g., 192.168.7.2 or 10.0.0.3)
   • Example configuration file

3. Host machine setup:

   • NetworkManager "Shared" method (easiest)
   • Manual iptables setup (for non-NM systems)
   • Required host IP address

4. Where to place overrides:

   • Kernel config for modules
   • Root overlay for NetworkManager configs
   • Example paths and files

Implementation Details

From #330 discussion, this involves:

• Loading dwc2 driver (kernel module or cmdline)
• Possibly: modules=sd-mod,usb-storage,ext4,dwc2,g_ether in cmdline.txt
• Or: adding to /etc/modules in root overlay
• NetworkManager connection file similar to licheerv's usb0.nmconnection

Benefits

• Much easier initial setup - no network scanning needed
• Provides reliable static IP for first login
• Useful for headless setups
• Emergency access if WiFi/Ethernet fails
• Follows existing pattern from licheerv docs

Location

This documentation should go in:

• configs/pi/README.md - add new section "USB Gadget Networking"
• Could reference licheerv docs as the detailed example
• Include Pi-specific kernel module details

Related

• Issue Feedback and questions #330
• Existing USB gadget docs in configs/allwinner/licheerv/README.md
• Similar feature requested for easier device discovery