Maybe try to do like this (according https://github.com/grml/live-boot-
grml/blob/master/debian/patches/26_support_dns_bootoption.patch):
1) +++ live-boot-grml/scripts/boot/9990-cmdline-old
leave only:
+dns=*)
+DNSSERVERS="${_PARAMETER#*=}"
+export DNSSERVERS
;;
2) in +++ live-boot-grml/scripts/boot/9990-grml-networking.sh make:
+# dns bootoption
+if [ -n "$DNSSERVERS" ]
+then
+	# disable any existing entries
+	if [ -r $RESOLVCONF ]
+	then
+		sed -i 's/nameserver/# nameserver/' $RESOLVCONF
+	fi
+       DNSSERVERS=`echo $DNSSERVERS| sed 's/,/ /g'`
+	for i in $DNSSERVERS
+	do
+		echo "nameserver $i" >> $RESOLVCONF
+	done
+fi
+