nixos/stage-1: install networkd link files

Renaming an interface must be done in stage-1: otherwise udev will
report the interface as ready and network daemons (networkd, dhcpcd,
etc.) will bring it up. Once up the interface can't be changed and the
renaming will fail.

Note: link files are read directly by udev, so they can be used even
without networkd enabled.

nixos/modules/system/boot/stage-1.nix

@@ -205,13 +205,22 @@ let
     ''; # */
 
 
+  # Networkd link files are used early by udev to set up interfaces early.
+  # This must be done in stage 1 to avoid race conditions between udev and
+  # network daemons.
   linkUnits = pkgs.runCommand "link-units" {
       allowedReferences = [ extraUtils ];
       preferLocalBuild = true;
-    } ''
+    } (''
       mkdir -p $out
       cp -v ${udev}/lib/systemd/network/*.link $out/
-    '';
+      '' + (
+      let
+        links = filterAttrs (n: v: hasSuffix ".link" n) config.systemd.network.units;
+        files = mapAttrsToList (n: v: "${v.unit}/${n}") links;
+      in
+        concatMapStringsSep "\n" (file: "cp -v ${file} $out/") files
+      ));
 
   udevRules = pkgs.runCommand "udev-rules" {
       allowedReferences = [ extraUtils ];