can-isotop: prepend kernel.makeFlags to makeFlags

pkgs/os-specific/linux/can-isotp/default.nix

@@ -12,16 +12,17 @@ stdenv.mkDerivation {
     rev = "21a3a59e2bfad246782896841e7af042382fcae7";
     sha256 = "1laax93czalclg7cy9iq1r7hfh9jigh7igj06y9lski75ap2vhfq";
   };
-
-  KERNELDIR = "${kernel.dev}/lib/modules/${kernel.modDirVersion}/build";
-  INSTALL_MOD_PATH = "\${out}";
+  makeFlags = kernel.makeFlags ++ [
+    "KERNELDIR=${kernel.dev}/lib/modules/${kernel.modDirVersion}/build"
+    "INSTALL_MOD_PATH=${placeholder "out"}"
+  ];
 
   buildPhase = ''
-    make modules
+    make modules $makeFlags
   '';
 
   installPhase = ''
-    make modules_install
+    make modules_install $makeFlags
   '';
 
   nativeBuildInputs = kernel.moduleBuildDependencies;