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

yellow/overlay/etc/aimee-os/writable-etc

@@ -1 +1,2 @@
+d burp
 d mosquitto

yellow/portage/target/etc/portage/env/app-backup/burp

@@ -0,0 +1 @@
+fowners() { :; }

yellow/portage/target/etc/portage/package.accept_keywords/burp

@@ -0,0 +1 @@
+app-backup/burp **

yellow/squashfs.exclude

@@ -0,0 +1,5 @@
+etc/burp/CA.cnf
+etc/burp/burp-server.conf
+etc/burp/clientconfdir
+usr/aarch64-unknown-linux-gnu
+var/spool/burp

yellow/squashfs.pseudo

@@ -0,0 +1,2 @@
+/etc/burp m 0750 498 498
+/etc/burp/burp.conf m 0640 498 498