ena: pass makeFlags from kernel

pkgs/os-specific/linux/ena/default.nix

···
   hardeningDisable = [ "pic" ];
 
   nativeBuildInputs = kernel.moduleBuildDependencies;
+  makeFlags = kernel.makeFlags;
 
   # linux 3.12
   NIX_CFLAGS_COMPILE = "-Wno-error=implicit-function-declaration";
···
 
   installPhase = ''
     runHook preInstall
-    strip -S ena.ko
+    $STRIP -S ena.ko
     dest=$out/lib/modules/${kernel.modDirVersion}/misc
     mkdir -p $dest
     cp ena.ko $dest/