arm64: flashrom.forChromebook only for ec

In order to write to the chromebook embedded controller flash we
need to use Google's fork of flashrom.  However for writing to the
chromebook CPU firmware flash (which is a different flash chip) we
can use mainline flashrom.  So let's do that.

src/platform/kevin/default.nix

@@ -143,11 +143,9 @@ in {
 
     ecfw-scripts = final.nixpkgsOnBuildForHost.callPackage (import ./ecfw/scripts.nix) {
       inherit (final) ectool ecfw;
-      flashrom-chromebook = final.flashrom;
+      flashrom-chromebook = prev.flashrom.override { forChromebook = true; };
     };
 
-    flashrom = prev.flashrom.override { forChromebook = true; };
-
     ectool = final.nixpkgsOnBuildForHost.callPackage (import ../../util/ectool { boardName = "kevin"; }) { };
 
     scripts = let