linux stdenv: Avoid assert false

On one hand, don't want to pass garbage that affects hash, on the other
hand footguns are bad.

Now, factored out the derivation so only need to pass in what is used.

pkgs/stdenv/linux/default.nix

@@ -37,22 +37,7 @@ rec {
 
 
   # Download and unpack the bootstrap tools (coreutils, GCC, Glibc, ...).
-  bootstrapTools = derivation {
-    name = "bootstrap-tools";
-
-    builder = bootstrapFiles.busybox;
-
-    args = [ "ash" "-e" ./scripts/unpack-bootstrap-tools.sh ];
-
-    tarball = bootstrapFiles.bootstrapTools;
-
-    inherit system;
-
-    # Needed by the GCC wrapper.
-    langC = true;
-    langCC = true;
-    isGNU = true;
-  };
+  bootstrapTools = import ./bootstrap-tools { inherit system bootstrapFiles; };
 
 
   # This function builds the various standard environments used during