register dev host as target's update source
usage: fx add-update-source [--addr ADDR] [--port PORT] [--name NAME] Configure target device to use a new update source. --name NAME Name the generated update source config NAME. --addr ADDR Specify the package server address explicitly. --port PORT The port at ADDR on which the package server is running (default: 8083). ADDR: Any IPv4 / IPv6 address or publicly reachable DNS name. Note: ADDR must be the address as-routable by the target device. If an link-local IPv6 address is provided, it must include a valid scope for the target (e.g. %ethp0014). If no ADDR is provided, the default address is assumed to be the address of the host machine as seen by the target from an SSH connection to the target device. NAME: If no name is supplied, the name defaults to "devhost", otherwise it contains the address given.