nrfutil device program Nordic Secure DFU.

Nordic Secure Device Firmware Update (DFU) is made over a UART serial connection.

A DFU package is required, see Generating DFU packages in the nRF Util for nRF5 SDK User Guide.

Nordic Secure DFU is the primary programming method for the nRF52840 dongle. The following procedure replaces the DFU over USB serial connection in the nRF Util for nRF5 SDK command.

List the devices with a DFU trigger interface, by running:
```
nrfutil device list --traits nordicDfu
```
If the device does not show up with the expected trait, it might need to be put into programming mode by holding in the reset button and reconnecting the dongle to the system.
Program new firmware to a device identified by its serial number, by running:
```
nrfutil device program --firmware dfu_package.zip --serial-number xxxxxxxxxxxx
```
The device is automatically put into application mode after programming. This behavior can be controlled using the --options flag, see:
```
nrfutil device program --help
```

Programming an nRF52840 Dongle over Nordic Secure DFU