fff-sysupgrade: install config via uci-defaults

Unfortunately the sysupgrade.conf is installed by base-files. The installed file
just contains examples and will be overwritten with this.

Signed-off-by: Tim Niemeyer <tim@tn-x.org>
Reviewed-by: Christian Dresel <fff@chrisi01.de>
Reviewed-by: Jan Kraus <mayosemmel@gmail.com>