Add the tool "nixos-typecheck" that can check an option declaration to:

- Enforce that an option declaration has a "defaultText" if and only if the
   type of the option derives from "package", "packageSet" or "nixpkgsConfig"
   and if a "default" attribute is defined.

 - Enforce that the value of the "example" attribute is wrapped with "literalExample"
   if the type of the option derives from "package", "packageSet" or "nixpkgsConfig".

 - Warn if a "defaultText" is defined in an option declaration if the type of
   the option does not derive from "package", "packageSet" or "nixpkgsConfig".

 - Warn if no "type" is defined in an option declaration.

nixos/modules/system/boot/kernel.nix

@@ -20,6 +20,7 @@ in
   options = {
 
     boot.kernelPackages = mkOption {
+      type = types.packageSet;
       default = pkgs.linuxPackages;
       # We don't want to evaluate all of linuxPackages for the manual
       # - some of it might not even evaluate correctly.

nixos/modules/system/boot/systemd-unit-options.nix

@@ -17,14 +17,15 @@ in rec {
 
   unitOption = mkOptionType {
     name = "systemd option";
-    merge = loc: defs:
+    typerep = "(systemdOption)";
+    merge = _module: loc: defs:
       let
         defs' = filterOverrides defs;
         defs'' = getValues defs';
       in
         if isList (head defs'')
         then concatLists defs''
-        else mergeOneOption loc defs';
+        else mergeOneOption _module loc defs';
   };
 
   sharedOptions = {