Add USB serial bridge transport for repeater firmware

[darrylhodgins]

Adds native USB CDC support to RS232Bridge for selected repeater targets with a second USB CDC interface.

Initially, includes these targets:

• RAK4631
• SenseCAP Solar

The existing serial bridge frame format is unchanged; only its physical transport changes.

• USB CDC 0 remains the normal MeshCore CLI/debug Serial port.
• USB CDC 1 carries the serial bridge stream.

For all other bridge targets, existing UART behaviour remains unchanged.

Closes #2958

Testing

Built and bench-tested on physical hardware:

• RAK_4631_repeater_bridge_usbserial
• SenseCap_Solar_repeater_bridge_usbserial

Verified that the CLI and bridge enumerate as separate USB serial ports and that bridge traffic works over the second port.

[enigmaspb]

Will it work if I connect two boards RAK4631 / RAK19007 with a USB cable?

[darrylhodgins]

Will it work if I connect two boards RAK4631 / RAK19007 with a USB cable?

It may be possible to have one of the boards act as a USB host, but as it stands at this point, both boards will just sit there waiting for a host to appear. At least for now, the UART approach is the way to go for that.

This initial PR is meant to provide the serial stream to a computer (in my case it's a simple Raspberry Pi).

I'll look into whether USB OTG or having one of the boards act as a USB host is possible, but I think that would belong in a separate PR.

[enigmaspb]

I see, that's a pity, I was hoping to avoid soldering. Thank you for your response.