create cdc-ncm module

examples/l2tp.nix

@@ -20,7 +20,7 @@ in rec {
   };
 
   imports = [
-#    ../modules/wlan.nix
+    ../modules/cdc-ncm
     ../modules/network
     ../modules/vlan
     ../modules/ssh

modules/cdc-ncm/default.nix

@@ -0,0 +1,22 @@
+{ config, ... }:
+{
+  config = {
+    kernel.config = {
+      USB_NET_HUAWEI_CDC_NCM = "y";
+      USB_USBNET = "y";
+      USB_SERIAL = "y";
+      USB_SERIAL_OPTION = "y";
+    };
+
+    # an ncm ethernet adaptor does
+    # * usb modeswitch
+    # * AT commands
+    # and then behaves like a link.
+
+    # we could identify by vid:pid but that's a bit awkward
+    # if there's more than one present.  IMEI?
+
+    # https://stackoverflow.com/questions/5477882/how-to-i-detect-whether-a-tty-belonging-to-a-gsm-3g-modem-is-a-data-or-control-p
+  };
+}
+