Provide a way to query only usb boards

[hgrecco]

In many cases I know that my arduino board is connected via USB (not network). It seems that is currently not possible to look only for usb boards using arduino-cli board list. This would be convenient to reduce the discovery time.

[flatsiedatsie]

This would also make me very happy. I have developed a small web-based tool around the Arduino-CLI, and this option would make things more predictable. In the interface users are prompted to plug in their arduino. It would be odd if network based devices could 'magically' show up without them having plugged anything into the USB port yet.

[cmaglie]

at the moment the network discovery is disabled. Will keep the issue open for when the network/pluggable discovery will be re-added.

see #344

[umbynos]

In the meantime we added back the network discovery, the defualt discovery timout is 1 second, but if you like you can specify it with --discovery-timeout flag. For example arduino-cli board list --discovery-timeout 0.1s this is really fast.