cros-update: use curl instead of wget to check devserver health.

BUG=chromium:784462
TEST=None

Change-Id: I5375daaca169976358ead00335b8a4e7a6f16ee9
Reviewed-on: https://chromium-review.googlesource.com/767293
Tested-by: Xixuan Wu <xixuan@chromium.org>
Trybot-Ready: Xixuan Wu <xixuan@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Xixuan Wu <xixuan@chromium.org>
1 file changed