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/bootstrap-tools/default.nix

@@ -0,0 +1,18 @@
+{ system, bootstrapFiles }:
+
+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;
+}