net-eth: ensure single-subsystem control

Setting NM_CONTROLLED is apparently not enough to disable
/etc/net handling of a particular interface; thanks sem@
for noticing the fortunate error messages in logs
and explaining this peculiarity to me.

features.in/net-eth/rootfs/image-scripts.d/50-net-eth

@@ -37,6 +37,7 @@ write_iface() {
 		echo "DISABLED=no"
 		echo "BOOTPROTO=$proto"
 		echo "NM_CONTROLLED=$NMCTL"
+		echo "DISABLED=$NMCTL"
 		echo "USE_IFPLUGD=yes"
 	} > "$dir/options"
 }