register dev host as target's update source
usage: fx add-update-source [--type=TYPE] [--name=NAME] Configure target device to use a new update source. When --type is devhost (the default), configure the target to use the dev host's address as seen from the target fuchsia device. When --type is localhost, configure the target to use 127.0.0.1 as its update source. --name=NAME Name the generated update source config NAME. Defaults to the config type.