build_patches: Remove additional config files preserved during upgrade

OpenWrt saves additional config (and other) files included with
list_changed_conffiles in /sbin/sysupgrade.

This is only active since openwrt-18.06, since before that it was
suppressed by missing opkg.

Since we expect to only save what is in sysupgrade.conf, remove the
additional file list (and thus regain pre-openwrt-18.06
behavior).

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>