nRF Util: DFU over a serial USB connection procedure

The nRF Util tool supports Device Firmware Update (DFU)s over a USB CDC ACM connection.

DFU over a serial USB connection is supported only for chips that have USB pins (for example, nRF52840).

Note: The USB port for the interface MCU is not connected to the USB pins on the chip. If you are using the interface MCU, you must perform a DFU over a serial UART connection.

Complete the following steps to perform the DFU:

Connect the DFU target to your computer.
If your DFU target is an nRF52840 Development Kit (DK), use the USB port marked nRF USB.
Run nrfutil dfu usb-serial to do a full DFU procedure over a USB CDC ACM connection.
You can see available options by entering the following command:
```
nrfutil dfu usb-serial --help
```

For example, enter the following command to perform a DFU procedure of the file app_dfu_package.zip over COM3 at 115200 bits per second:

nrfutil dfu usb-serial -pkg app_dfu_package.zip -p COM3 -b 115200