Fuchsia: Reset all SDK timestamps to current time.

Ninja has a quirk that makes it blind to incremental source file
changes, so long as the source files' timestamp precedes the last
build time. As a result Ninja was not correctly picking up changes
in SDK rolls, resulting in an unbootable combination of fresh and stale
build files.

This CL effectively implements a recursive "touch" on the SDK files,
which gives an unambiguous change signal to Ninja.

Change-Id: I6b5e15a2b4404fcc7a2f52ebf76f75f80a0a6605
Reviewed-on: https://chromium-review.googlesource.com/1025376
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553267}

build/fuchsia/update_sdk.py

@@ -43,6 +43,16 @@ def Cleanup(path):
     os.remove(hash_file)
 
 
+# Updates the modification timestamps of |path| and its contents to the
+# current time.
+def UpdateTimestampsRecursive(path):
+  for root, dirs, files in os.walk(path):
+    for f in files:
+      os.utime(os.path.join(root, f), None)
+    for d in dirs:
+      os.utime(os.path.join(root, d), None)
+
+
 def main():
   if len(sys.argv) != 1:
     print >>sys.stderr, 'usage: %s' % sys.argv[0]
@@ -92,6 +102,8 @@ def main():
   with open(hash_filename, 'w') as f:
     f.write(sdk_hash)
 
+  UpdateTimestampsRecursive(output_dir)
+
   return 0