build kernel only once for multiple outputs

e.g. vmlinux + zImage

modules/arch/arm.nix

@@ -4,6 +4,7 @@
     kernel.config = {
       OF = "y";
     };
+    kernel.makeTargets = ["arch/arm/boot/zImage"];
     hardware.ram.startAddress = lim.parseInt "0x40000000";
     system.outputs.u-boot = pkgs.ubootQemuArm;
   };

modules/kernel.nix

@@ -9,6 +9,7 @@ let
   inherit (pkgs.pseudofile) dir symlink;
   inherit (pkgs.liminix.networking) address interface;
   inherit (pkgs.liminix.services) bundle;
+  inherit (pkgs) liminix;
 
   type_service = pkgs.liminix.lib.types.service;
 
@@ -41,11 +42,25 @@ in {
           };
         '';
       };
+      makeTargets = mkOption {
+        type = types.listOf types.str;
+      };
     };
   };
   config = {
+    system.outputs =
+      let k = liminix.builders.kernel.override {
+            inherit (config.kernel) config src extraPatchPhase;
+            targets = config.kernel.makeTargets;
+          };
+      in {
+        kernel = k.vmlinux;
+        zimage = k.zImage;
+      };
+
     kernel = rec {
       modular = true; # disabling this is not yet supported
+      makeTargets = ["vmlinux"];
       config = {
         IKCONFIG = "y";
         IKCONFIG_PROC = "y";

modules/outputs.nix

@@ -99,26 +99,18 @@ in
   };
   config = {
     system.outputs = rec {
-      kernel = liminix.builders.kernel.override {
-        inherit (config.kernel) config src extraPatchPhase;
-      };
-      zimage = liminix.builders.kernel.override {
-        targets = ["arch/arm/boot/zImage"];
-        inherit (config.kernel) config src extraPatchPhase;
-      };
       dtb = liminix.builders.dtb {
         inherit (config.boot) commandLine;
         dts = config.hardware.dts.src;
         includes = config.hardware.dts.includes ++ [
-          "${kernel.headers}/include"
+          "${o.kernel.headers}/include"
         ];
       };
       uimage = liminix.builders.uimage {
         commandLine = concatStringsSep " " config.boot.commandLine;
         inherit (config.hardware) loadAddress entryPoint;
         inherit (config.boot) imageFormat;
-        inherit kernel;
-        inherit dtb;
+        inherit (o) kernel dtb;
       };
       rootdir =
         let