dev: add cros_fetch_image script to download prebuilt images
dev: add cros_fetch_image script to download prebuilt images

This script provides a simple front-end to download images from
gs://chromeos-image-archive and gs://chromeos-releases. The
images are placed in the usual ~/trunk/src/build/${BOARD}
directory and symlinked for use with image_to_usb (and similar).

BUG=chromium-os:27665
TEST=Several manual runs fetching different boards and versions.
CQ-DEPEND=I532591b17ce75c182bef9bf9c6fb6711bb812bb9

Change-Id: I100c437666d3f1be343e0dd228ecfd5e2f8d62f9
Reviewed-on: https://gerrit.chromium.org/gerrit/21977
Reviewed-by: Chris Wolfe <4f23406e03c8ffaccefe8744212d3543c02e8d87@chromium.org>
Tested-by: Chris Wolfe <4f23406e03c8ffaccefe8744212d3543c02e8d87@chromium.org>
Commit-Ready: Chris Wolfe <4f23406e03c8ffaccefe8744212d3543c02e8d87@chromium.org>

coreos/dev-util,mgireesh05/dev-util,coreos/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,marineam/coreos-dev-util,coreos/dev-util,mgireesh05/dev-util