We no longer copy service directories into `/etc/coreos-home-server` if
these have not had their respective `spec.bu` files included; these
directories are not needed in these cases, and would be erroneously
considered as eligible in subsequent `coreos-home-server-update`
invocations.
This commit contains a fairly large diff for a fairly small change:
moving the `config/common` directory to `host/base` to better reflect
its intended use, and promoting `config/service` to the root directory.
These changes unlock some improvements in `coreos-home-server-update`
processes, which will (assuming `/etc/coreos-home-server/base` exists)
keep host-wide systemd services in sync in addition to service-specific
ones.
Changes have been make to the `Makefile` and a few other places where
`config/common` was referenced, but most of this work is renames that
are not intended to break compatibility with new or running servers.
