Check wan_ifname before using it:

Bug on Unify:  Non-existing eth1 is assigned by OpenWRT

files/lib/freifunk/upgrade/initial/011-freifunk-network

@@ -8,6 +8,13 @@ local board="$(ar71xx_board_name)"
 local wan_ifname="$(uci get network.wan.ifname)"
 local main_mac="$(get_main_address)"
 
+if [ -n "$wan_ifname" ]; then
+  ifconfig $wan_ifname
+  if [ $? -ne 0 ]; then
+    wan_ifname=""
+  fi
+fi
+
 uci -q batch <<EOF
 delete network.lan
 delete network.wan6
@@ -21,8 +28,10 @@ set network.wan.auto='1'
 set network.freifunk='interface'
 EOF
 
+
+
 # Falls kein WAN-Interface vorhanden, nutze einziges LAN-Interface
-if [ -z "$wan_ifname" ]
+if [ -n "$wan_ifname" ]
 then		
 		uci set network.freifunk.ifname="bat0"
 		uci set network.wan.ifname="$lan_ifname"