[platform] Move the stateful update from wget to curl

wget is removed from the rootfs crbug.com/781538 change the
stateful update to use curl rather than wget

BUG=chromium:785487
TEST=ran unittests, will run trybot job before submit.

Change-Id: I0afc1b3e3f9520cc20f8ff65d38712b4957e010b
Reviewed-on: https://chromium-review.googlesource.com/773114
Commit-Ready: Keith Haddow <haddowk@chromium.org>
Tested-by: Keith Haddow <haddowk@chromium.org>
Reviewed-by: David Riley <davidriley@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
(cherry picked from commit 431d3aca8bccc38d2affebc1d8c975b2c2db16d5)
Reviewed-on: https://chromium-review.googlesource.com/900265
Reviewed-by: Keith Haddow <haddowk@chromium.org>
Commit-Queue: Keith Haddow <haddowk@chromium.org>
Trybot-Ready: Keith Haddow <haddowk@chromium.org>

stateful_update

diff --git a/stateful_update b/stateful_update
index 3e12337..e76d058 100755
--- a/stateful_update
+++ b/stateful_update
@@ -41,7 +41,7 @@
 
 download_stateful_payload(){
   # Download and unzip directories onto the stateful partition.
-  eval "wget -qS -T 300 -O - \"${stateful_update_url}\"" |
+  eval "curl --silent --max-time 300 \"${stateful_update_url}\"" |
       tar --ignore-command-error --overwrite --directory=${STATEFUL_DIR} -xz
 }