fetchurlBoot: Use Nix's builtin fetchurl function

This removes the need for curl in bootstrapTools, and enables https
for bootstrap tarballs.

pkgs/build-support/fetchurl/boot.nix

@@ -0,0 +1,20 @@
+let mirrors = import ./mirrors.nix; in
+
+{ system }:
+
+{ url ? builtins.head urls
+, urls ? []
+, sha256
+}:
+
+import <nix/fetchurl.nix> {
+  inherit system sha256;
+
+  url =
+    # Handle mirror:// URIs. Since <nix/fetchurl.nix> currently
+    # supports only one URI, use the first listed mirror.
+    let m = builtins.match "mirror://([a-z]+)/(.*)" url; in
+    if m == null then url
+    else builtins.head (mirrors.${builtins.elemAt m 0}) + (builtins.elemAt m 1);
+
+}

pkgs/stdenv/linux/default.nix

@@ -80,9 +80,9 @@ rec {
           '';
         shell = "${bootstrapTools}/bin/bash";
         initialPath = [bootstrapTools];
-        fetchurlBoot = import ../../build-support/fetchurl {
+
-          stdenv = stage0.stdenv;
+        fetchurlBoot = import ../../build-support/fetchurl/boot.nix {
-          curl = bootstrapTools;
+          inherit system;
         };
 
         cc = if isNull gccPlain
@@ -333,7 +333,6 @@ rec {
       awk --version
       grep --version
       gcc --version
-      curl --version
 
       ldlinux=$(echo ${bootstrapTools}/lib/ld-linux*.so.?)
       export CPP="cpp -idirafter ${bootstrapTools}/include-glibc -B${bootstrapTools}"

pkgs/stdenv/linux/make-bootstrap-tools.nix

@@ -10,13 +10,6 @@ rec {
     aclSupport = false;
   });
 
-  curlMinimal = curl.override {
-    http2Support = false;
-    zlibSupport = false;
-    sslSupport = false;
-    scpSupport = false;
-  };
-
   busyboxMinimal = busybox.override {
     useMusl = true;
     enableStatic = true;
@@ -84,8 +77,6 @@ rec {
         cp -d ${gnumake}/bin/* $out/bin
         cp -d ${patch}/bin/* $out/bin
         cp ${patchelf}/bin/* $out/bin
-        cp ${curlMinimal}/bin/curl $out/bin
-        cp -d ${curlMinimal}/lib/libcurl* $out/lib
 
         cp -d ${gnugrep.pcre}/lib/libpcre*.so* $out/lib # needed by grep