new kernelAtleast option at lib/kernel-version.nix

Issues a warning when the kernel version is below a threshhold.

lib/kernel-version.nix

@@ -0,0 +1,31 @@
+{ config, lib, ... }:
+with lib;
+let
+  cfg = config;
+in
+{
+  options.kernelAtleast = mkOption {
+    type = types.listOf types.optionSet;
+    options =
+      [ { version = mkOption {
+            type = types.str;
+            example = "4.4";
+            description =
+              "Issue warning when kernel version is below this number.";
+          };
+          msg = mkOption {
+            type = types.str;
+            example = "";
+          };
+        }
+      ];
+  };
+
+  config.warnings = builtins.concatLists (map
+    (x: if (builtins.compareVersions cfg.boot.kernelPackages.kernel.version x.version) == -1
+        then [ "${x.msg} (${cfg.boot.kernelPackages.kernel.version} < ${x.version})" ]
+        else [ ]
+    ) cfg.kernelAtleast
+  );
+
+}