AU dev-server: Add a sha256 hash to the updatecheck response.
AU dev-server: Add a sha256 hash to the updatecheck response.

BUG=6580
TEST=tested with an update_engine that takes SHA-256.

Change-Id: Id7d507fe65f8eb32d77b14b76b80d3d7ba52182b

Review URL: http://codereview.chromium.org/3455018

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