`dashboard` wants a `read_8` backend

[mkeeter]

matt@niles ~ () $ h --ip=fe80::aa40:25ff:fe04:06c7%axf9 -a build-gimlet-d-image-default.zip dashboard
humility: connecting to fe80::aa40:25ff:fe04:6c7%5
humility dashboard failed: Could not find `udprpc` task in this image. Only -dev and -lab images include `udprpc`; are you running a production image?

#491 implemented a network-compatible backend for humility sensors; we should do the same for humility dashboard. We wouldn't be able to read / set power state, but that's fine.