nixos i18n: add option to set console keymap from xkb

Close #9675. The expression was refactored heavily by vcunat.

nixos/modules/config/i18n.nix

···
         '';
       };
 
+      consoleUseXkbConfig = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          If set, configure the console keymap from the xserver keyboard
+          settings.
+        '';
+      };
+
       consoleKeyMap = mkOption {
         type = mkOptionType {
           name = "string or path";
···
   ###### implementation
 
   config = {
+
+    i18n.consoleKeyMap = with config.services.xserver;
+      mkIf config.i18n.consoleUseXkbConfig
+        (pkgs.runCommand "xkb-console-keymap" { preferLocalBuild = true; } ''
+          '${pkgs.ckbcomp}/bin/ckbcomp' -model '${xkbModel}' -layout '${layout}' \
+            -option '${xkbOptions}' -variant '${xkbVariant}' > "$out"
+        '');
 
     environment.systemPackages =
       optional (config.i18n.supportedLocales != []) glibcLocales;