[build] Generate last-update in GN from jiri history, not from jiri hook

The time stamp of the last `jiri update` is visible in the
`//.jiri_root/update/update_history/` directory.  So rather than
touching the source tree in a hook that runs at the end of `jiri
update`, just generate the timestamp file in the GN build as a
function of the files `jiri update` writes.

Bug: BLD-200 #comment Stop writing //garnet/last-update file in jiri hook
Test: CQ
Change-Id: I41c1448b0fed8d0cb3985ee032d1b7d4c6209a36

.gitignore

@@ -23,7 +23,6 @@ tags
 target
 go/pkg
 *.orig
-last-update
 /.cipd
 tools/cipd.gni
 .idea/

BUILD.gn

@@ -4,16 +4,28 @@
 
 import("//build/package.gni")
 
+jiri_snapshot_file = "//.jiri_root/update_history/latest"
+last_update_file = "$target_gen_dir/last-update.txt"
+
 package("build_info") {
   deprecated_system_image = true
+  deps = [ ":last-update.txt" ]
   resources = [
     {
       dest = "build/last-update"
-      path = rebase_path("last-update")
+      path = last_update_file
     },
     {
       dest = "build/snapshot"
-      path = rebase_path("//.jiri_root/update_history/latest")
+      path = jiri_snapshot_file
     },
   ]
 }
+
+action("last-update.txt") {
+  visibility = [ ":*" ]
+  sources = [ jiri_snapshot_file ]
+  outputs = [ last_update_file ]
+  script = "tools/gen-last-update"
+  args = rebase_path(sources + outputs, root_build_dir)
+}

manifest/minimal

@@ -7,9 +7,6 @@
              gerrithost="https://fuchsia-review.googlesource.com"/>
   </projects>
   <hooks>
-    <hook name="build-update"
-          project="garnet"
-          action="tools/update.sh"/>
     <hook name="cipd-update"
           project="garnet"
           action="tools/cipd-update.sh"/>

tools/update.sh → tools/gen-last-update

-#!/bin/bash
-# Copyright 2017 The Fuchsia Authors. All rights reserved.
+#!/bin/sh
+# Copyright 2018 The Fuchsia Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-date +%FT%T%z > last-update
+exec readlink "$1" > "$2"