Is your feature request related to a problem? Please describe.
With the upcoming polling support in #1672, it would be useful to allow changes to the configuration without restarting the service.

Describe the solution you'd like
We should add a SIGHUP handler to trigger a config reload. If RAUC is currently busy, it should postpone the reload until the ongoing operation has completed. After the config reload, the slot status als needs to be reloaded.

Describe alternatives you've considered
Simply restarting the service would also work, but that would potentially interrupt an ongoing installation.