WIP refactor

common/boot/default.nix

@@ -0,0 +1,35 @@
+{ config, lib, ... }:
+let
+  ccfg = import ../config.nix;
+  cfg = config.${ccfg.custom_config_key}.boot;
+in
+{
+  imports = [
+    ./grub.nix
+    ./systemd.nix
+  ];
+  config = {
+    assertions = [
+      (
+        let
+          enabledBootloaders = lib.filter (x: x.enabled) [
+            {
+              name = "systemd";
+              enabled = cfg.systemd.enable;
+            }
+            {
+              name = "grub";
+              enabled = cfg.grub.enable;
+            }
+          ];
+        in
+        {
+          assertion = lib.length enabledBootloaders <= 1;
+          message =
+            "Only one bootloader can be enabled at a time. Enabled: "
+            + lib.concatStringsSep ", " (map (x: x.name) enabledBootloaders);
+        }
+      )
+    ];
+  };
+}