Changing physical service config is hard

[andrewn]

Changing the physical service config (so you can control different components) currently requires either:

• renaming one of the sample config files to be called physical-config.json. But then, git pull will overwrite this.
• ssh'ing into the Pi and editing /etc/systemd/system/physical.service to include the CONFIG_PATH env var pointing to the file.

Neither approach is ideal. We could:

• load the config file from somewhere like /opt/radiodan/config/physical.json (if it exists) and mount over samba so it can be edited easily
• allow components to be loaded dynamically like the serial service so that apps can request the components they need using WebSocket messages.