yellow: Install BURP

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.

yellow/install.packages

@@ -1 +1,2 @@
+app-backup/burp
 app-containers/podman