[slik]

Route External RF module over USB-VCP

Adds "External module" as an USB-VCP option. When it's selected, the External RF protocol driver (CRSF, MAVLink, …) uses the USB serial port instead of the module bay. A PC can then act as a module with selected protocol, both extracting channel values from packets and sending back telemetry.

It should be useful for integration with GCS software or simulators:

• Applications can use all 16 (depends on the protocol) channels
• Better support on all OSes, as some of them have various USB HID limitations (e.g., max axes count on Android)
• Easier mixer configuration, as it is not required to configure axes and buttons captured via USB HID
• Bi-directional communication allows GCS/simulator software send back telemetry values
• Protocol-agnostic, allows GCS to communicate with handset using the same protocol it uses for communication with autopilot (CRSF or MAVLink), no need for translation

Usage: Radio Setup → Hardware → USB-VCP → External module, then pick the protocol under Model → External RF as usual.

Changes

• module_port: when VCP is in UART_MODE_EXT_MODULE, the external module's serial port lookup returns a full-duplex USB port.
• usbd_cdc: implements the serial driver calls the module drivers need (sendBuffer, txCompleted, RX buffer copy/clear, idle callback, getBufferedBytes), with guards for when no host is connected.
• serial: VCP is included in initSerialPorts(), and switching it to this mode restarts the external module pulses.
• gui_common: the mode is selectable on VCP even on radios without a spare UART for an external module.