Fix relative symlinks to /usr

Some config files are relative links to /usr, like
20-systemd-ssh-proxy.conf. These relative links are
broken in /usr/share/factory/etc unless /usr is linked
to /usr/share/factory/usr

Author: p1gp1g

mkosi.finalize

@@ -6,3 +6,4 @@ set -e
 # systemd-tmpfiles to symlink individual directories from it to /etc.
 mkdir -p "$BUILDROOT/usr/share/factory/"
 cp --archive --no-target-directory --update=none "$BUILDROOT/etc" "$BUILDROOT/usr/share/factory/etc"
+ln -s /usr "$BUILDROOT/usr/share/factory/usr"