cros_portage_upgrade -- Ignore chromeos-base packages early.
cros_portage_upgrade -- Ignore chromeos-base packages early.

BUG=none
TEST=ran it

Change-Id: I4f7cf85f385b719ee8a9032545fbdb7c0ab97f81

Review URL: http://codereview.chromium.org/6903041

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