Make it easier to build for non-official LineageOS devices

[danielfullmer]

The update_device_dirs.py script can fetch additional repos for a device not included in the list of official supported devices (from hudson). e.g. ./update_device_dirs.py --branch lineage-18.1 sony_huashan. However, it currently only fetches from https://github.com/LineageOS, and places its output in the existing device-dirs.json file. We should make it easy to have it output something that could be immediately included in an end-user's config.

https://github.com/danielfullmer/robotnix/issues/149