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;
   };