Refactor cros_update.py to make it possible to support upgrades.
Refactor cros_update.py to make it possible to support upgrades.

This is just a refactor, no real functional changes. The goal is to
move the step that finds the latest versions of the packages to be the
last so that it can also perform upgrades.

BUG=chromium-os:13591
TEST=ran it

Change-Id: I2f6ce1f0fc7bc0890f70d40d81c57fc32475e745

Review URL: http://codereview.chromium.org/6718037

bpsinc-native/src_third_party_chromite,coreos/chromite,chadversary/chromiumos.chromite,coreos/chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,zhang0137/chromite