I have a custom dongle that connects to the PC through a USB-UART chip. It is based on the nRF51822. I have another board based on the nRF52832. This board needs to be able to receive a DFU over BLE through the dongle and no other means for functional reasons. How do I perform this over the COM port using nrfutil? Is there a command for this? The dongle has firmware on it that cannot be overwritten. So only the COM port must be utilised.