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.

[hickey]

I have just published #3020 which is a similar effort. Instead of trying shoehorn the code in to the repeater code base I have created a USBSerial bridge code that is in src/helpers/bridges that gets activated by defining WITH_USB_SERIAL_BRIDGE. There is secondary define that specifies the serial stream to use for the bridge transport. In theory this would support this author's use case, but I am not familiar enough with platforms that support multiple USB serial ports to confirm if this is true or not.

In addition, I have added the TCP bridge to #3020 in a similar manner. Further details are described in #3020.

I have also added code to return more descriptive values for the get bridge.type CLI command that is not present in this PR.

[hickey]

Killed off #3020 as it was not based on the dev branch.

Reopened the PR as #3029 and there is a feature request issue created as #3028.