Don't allow hardlinks in tarball.

These errors on extraction if you don't have the original targets
of the links on the image (i.e. if the stateful_partition gets wiped).

Change-Id: Id5d3f770830de6165613d506733c04edefccd3fb

BUG=chromium-os:9103
TEST=Ran update, wiped partition, ran stateful_update and ran bvt's.

Review URL: http://codereview.chromium.org/4882006

cros_generate_stateful_update_payload.py

@@ -50,6 +50,7 @@ def GenerateStatefulPayload(image_path, output_directory, logger):
                              '-czf',
                              output_gz,
                              '--directory=%s' % stateful_dir,
+                             '--hard-dereference',
                              '--transform=s,^dev_image,dev_image_new,',
                              '--transform=s,^var,var_new,',
                              'dev_image',