Move stdenv bootstrap binaries out of the tree

Commit 986f36194650e2a41451cbfb9f29ce1c66a62df3 started to use
<nix/fetchurl.nix> to "download" the bootstrap binaries from the
Nixpkgs tree, using the file:/// scheme. This has really bad
consequences:

* It makes any derivation depend on the path of the Nixpkgs tree. So
evaluating a package will produce a different .drv file when run
from different locations. No wonder Hydra evaluation has been so
slow lately: for every Nixpkgs evaluation, it had to create tens of
thousands of .drv files, even if nothing had changed.

* It requires the builder to have file system access to the Nixpkgs
tree. So if your tree is in your home directory, the stdenv
bootstrap would probably fail.

So now the binaries are downloaded from tarballs.nixos.org.

Also dropped PowerPC "support".

pkgs/stdenv/linux/bootstrap/armv5tel/default.nix

···
-let fetch = { file, sha256 }:
-    let
-      nixFetchurl = import <nix/fetchurl.nix>;
-      args = {
-        url = "file://${builtins.toString ./.}/${file}";
-        inherit sha256;
-        executable = true;
-      };
-    in if (builtins.functionArgs nixFetchurl) ? executable
-      then nixFetchurl args
-      else derivation {
-        name = file;
-        builder = "/bin/sh";
-
-        system = builtins.currentSystem;
-
-        args = [ "-c" "echo $message; exit 1" ];
-
-        message = ''
-          Sorry, this version of nix cannot download all of the bootstrap tools.
-          Please download ${args.url}, make it executable, add it to the store
-          with `nix-store --add', and try again.
-        '';
-
-        outputHashAlgo = "sha256";
-
-        outputHash = args.sha256;
+let
 
-        outputHashMode = "recursive";
+  fetch = { file, sha256 }: import <nix/fetchurl.nix> {
+    url = "http://tarballs.nixos.org/stdenv-linux/armv5tel/r18744/${file}";
+    inherit sha256;
+    executable = true;
+  };
 
-        preferLocalBuild = true;
-      };
 in {
   sh = fetch {
     file = "sh";

pkgs/stdenv/linux/bootstrap/i686/default.nix

···
 let
-  fetch = { file, sha256 }:
-    let
-      nixFetchurl = import <nix/fetchurl.nix>;
-      args = {
-        url = "file://${builtins.toString ./.}/${file}";
-        inherit sha256;
-        executable = true;
-      };
-    in if (builtins.functionArgs nixFetchurl) ? executable
-      then nixFetchurl args
-      else derivation {
-        name = file;
-        builder = "/bin/sh";
 
-        system = builtins.currentSystem;
-
-        args = [ "-c" "echo $message; exit 1" ];
-
-        message = ''
-          Sorry, this version of nix cannot download all of the bootstrap tools.
-          Please download ${args.url}, make it executable, add it to the store
-          with `nix-store --add', and try again.
-        '';
+  fetch = { file, sha256 }: import <nix/fetchurl.nix> {
+    url = "http://tarballs.nixos.org/stdenv-linux/i686/r24519/${file}";
+    inherit sha256;
+    executable = true;
+  };
 
-        outputHashAlgo = "sha256";
-
-        outputHash = args.sha256;
-
-        outputHashMode = "recursive";
-
-        preferLocalBuild = true;
-      };
 in {
   sh = fetch {
     file = "sh";
···
   cpio = fetch {
     file = "cpio";
     sha256 = "046if3aqqramyhrn2yxrjf4bfkl8x1bcqxhvi7ml9nrv9smx8irg";
-  };
-
-  ln = fetch {
-    file = "ln";
-    sha256 = "06vr474i3x55p0rnqa87yx7dzf4qdfpfg201mks39id43cjm9f8j";
   };
 
   curl = fetch {

pkgs/stdenv/linux/bootstrap/loongson2f/default.nix

···
 let
-  fetch = { file, sha256 }:
-    let
-      nixFetchurl = import <nix/fetchurl.nix>;
-      args = {
-        url = "file://${builtins.toString ./.}/${file}";
-        inherit sha256;
-        executable = true;
-      };
-    in if (builtins.functionArgs nixFetchurl) ? executable
-      then nixFetchurl args
-      else derivation {
-        name = file;
-        builder = "/bin/sh";
 
-        system = builtins.currentSystem;
+  fetch = { file, sha256 }: import <nix/fetchurl.nix> {
+    url = "http://tarballs.nixos.org/stdenv-linux/loongson2f/r22849/${file}";
+    inherit sha256;
+    executable = true;
+  };
 
-        args = [ "-c" "echo $message; exit 1" ];
-
-        message = ''
-          Sorry, this version of nix cannot download all of the bootstrap tools.
-          Please download ${args.url}, make it executable, add it to the store
-          with `nix-store --add', and try again.
-        '';
-
-        outputHashAlgo = "sha256";
-
-        outputHash = args.sha256;
-
-        outputHashMode = "recursive";
-
-        preferLocalBuild = true;
-      };
 in {
   sh = fetch {
     file = "sh";

pkgs/stdenv/linux/bootstrap/powerpc/default.nix

···
-let
-  fetch = { file, sha256 }:
-    let
-      nixFetchurl = import <nix/fetchurl.nix>;
-      args = {
-        url = "file://${builtins.toString ./.}/${file}";
-        inherit sha256;
-        executable = true;
-      };
-    in if (builtins.functionArgs nixFetchurl) ? executable
-      then nixFetchurl args
-      else derivation {
-        name = file;
-        builder = "/bin/sh";
-
-        system = builtins.currentSystem;
-
-        args = [ "-c" "echo $message; exit 1" ];
-
-        message = ''
-          Sorry, this version of nix cannot download all of the bootstrap tools.
-          Please download ${args.url}, make it executable, add it to the store
-          with `nix-store --add', and try again.
-        '';
-
-        outputHashAlgo = "sha256";
-
-        outputHash = args.sha256;
-
-        outputHashMode = "recursive";
-
-        preferLocalBuild = true;
-      };
-in {
-  bash = fetch {
-    file = "bash";
-    sha256 = "0zss8im6hbx6z2i2wxn1554kd7ggdqdli4xk39cy5fchlnz9bqpp";
-  };
-
-  bzip2 = fetch {
-    file = "bzip2";
-    sha256 = "01ylj8x7albv6k9sqx2h1prsazh4d8y22nga0pwai2bnns0q9qdg";
-  };
-
-  cp = fetch {
-    file = "cp";
-    sha256 = "0d7xbzrv22bxgw7w9b03rakirna5zfvr9gzwm7ichd2fh634hvgl";
-  };
-
-  curl = fetch {
-    file = "curl.bz2";
-    sha256 = "17c25dfslw3qkjlcmihpbhn3x4kj9pgkslizv89ggnki7iiy4jgh";
-  };
-
-  tar = fetch {
-    file = "tar.bz2";
-    sha256 = "132ylqwz02hw5njqx7wvj4sxpcrllx8b8b3a00rlv6iad671ayyr";
-  };
-
-  staticToolsURL = {
-    url = http://tarballs.nixos.org/stdenv-linux/powerpc/r9828/static-tools.tar.bz2;
-    sha1 = "e4d1680e3dfa752e49a996a31140db53b10061cb";
-  };
-
-  binutilsURL = {
-    url = http://tarballs.nixos.org/stdenv-linux/powerpc/r9828/binutils.tar.bz2;
-    sha1 = "2609f4d9277a60fcd178395d3d49911190e08f36";
-  };
-
-  gccURL = {
-    url = http://tarballs.nixos.org/stdenv-linux/powerpc/r9828/gcc.tar.bz2;
-    sha1 = "71d79d736bfef6252208fe6239e528a591becbed";
-  };
-
-  glibcURL = {
-    url = http://tarballs.nixos.org/stdenv-linux/powerpc/r9828/glibc.tar.bz2;
-    sha1 = "bf0245e16235800c8aa9c6a5de6565583a66e46d";
-  };
-}

pkgs/stdenv/linux/default.nix

···
   bootstrapFiles =
     if system == "i686-linux" then import ./bootstrap/i686
     else if system == "x86_64-linux" then import ./bootstrap/x86_64
-    else if system == "powerpc-linux" then import ./bootstrap/powerpc
     else if system == "armv5tel-linux" then import ./bootstrap/armv5tel
     else if system == "armv6l-linux" then import ./bootstrap/armv6l
     else if system == "armv7l-linux" then import ./bootstrap/armv6l
···
       then [ ./scripts/unpack-bootstrap-tools-arm.sh ]
       else [ ./scripts/unpack-bootstrap-tools.sh ];
 
+    # FIXME: get rid of curl.
     inherit (bootstrapFiles) bzip2 mkdir curl cpio;
 
     tarball = import <nix/fetchurl.nix> {