Don't re-generate packages if packages have already been generated.
Don't re-generate packages if packages have already been generated.

By relying on the buildbot to generate packages for us, we save
roughly 13 minutes (22->9 minutes) in autotest staging time. In
order to remain backwards compatible, we check for the existence
of packages.checksum as it is not present until packages have been
generated.

BUG=chromium-os:27285
TEST=Unittests + pyflakes + pylint

Change-Id: I72c031920be6467a4d2941c1181c1abcb05551b2
Reviewed-on: https://gerrit.chromium.org/gerrit/17900
Tested-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Reviewed-by: Chris Masone <98e3d65c6b0e9e2391eb4c2e8b1e572b77e544ed@chromium.org>
Commit-Ready: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>

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