Enhance cros_update to support --upgrade.
Enhance cros_update to support --upgrade.

Under this option, packages that live in portage or portage-stable are actually
upgraded and a new git commit is generated.

BUG=chromium-os:13591
TEST=ran it

Change-Id: I2c065be3dcf041244954bcb3706e4c95dffa3189

Review URL: http://codereview.chromium.org/6749043

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