nixos/kernel: replace boot.isContainer with boot.kernel.enable

nixos/modules/system/activation/top-level.nix

@@ -27,7 +27,7 @@ let
 
       # Containers don't have their own kernel or initrd.  They boot
       # directly into stage 2.
-      ${optionalString (!config.boot.isContainer) ''
+      ${optionalString config.boot.kernel.enable ''
         if [ ! -f ${kernelPath} ]; then
           echo "The bootloader cannot find the proper kernel image."
           echo "(Expecting ${kernelPath})"

nixos/modules/system/boot/kernel.nix

@@ -20,6 +20,9 @@ in
   ###### interface
 
   options = {
+    boot.kernel.enable = mkEnableOption (lib.mdDoc "the Linux kernel. This is useful for systemd-like containers which do not require a kernel.") // {
+      default = true;
+    };
 
     boot.kernel.features = mkOption {
       default = {};
@@ -258,7 +261,7 @@ in
           ];
       })
 
-      (mkIf (!config.boot.isContainer) {
+      (mkIf config.boot.kernel.enable {
         system.build = { inherit kernel; };
 
         system.modulesTree = [ kernel ] ++ config.boot.extraModulePackages;

nixos/modules/virtualisation/container-config.nix

@@ -7,6 +7,8 @@ with lib;
   config = mkIf config.boot.isContainer {
 
     # Disable some features that are not useful in a container.
+    boot.kernel.enable = false;
+
     nix.optimise.automatic = mkDefault false; # the store is host managed
     powerManagement.enable = mkDefault false;
     documentation.nixos.enable = mkDefault false;