We'll use BURP for backups, just like the old Home Assistant server.
Note that Portage cannot correctly set the ownership of files when
installing to an alternate $ROOT. To work around this, we replace the
`fowners` function for the *app-backup/burp* ebuild with a no-op, and
then set the permissions of the relevant files using SquashFS
pseudo-file definitions. Relatedly, we're omitting the files and
directories used by the server-side of BURP.
The default Home Assistant configuration does not allow running a
reverse proxy in front of the application. To enable this, the
`use_x_forwarded_for` and `trusted_proxies` options have to be set.
Since we want `/var/lib/homeassistant` to be a Btrfs subvolume, we can't
simply include the necessary files in the correct location in the rootfs
image. Instead, we must define "copy tree" (`C`) actions for
`systemd-tmpfiles` to copy them from `/usr/share/factory`.
Unfortunately, `systemd-tmpfiles` considers `v` and `C` actions
conflicting, and thus will not copy the directory contents recursively.
Each file has to be listed explicitly.
Dustin