fix: restore foris config from snapshots

Hotfixes https://gitlab.nic.cz/turris/turris-os-packages/-/issues/657

collect/sentinel/sentinel-firewall/files/functions.sh

-. /lib/functions.sh
+#!/bin/sh
+## sentinel-firewall.sh
+#
+# set of firewall rules handling functions intended to be sourced and reused
+# in sentinel-related scripts
+
+# source OpenWrt functions if not sourced yet
+command -v config_load > /dev/null || . /lib/functions.sh
+
 
 # This function enables given option on firewall zone wan unless it is already set
 # to some value.