Require staged upgrades to be in cros_portage_upgrade target list.
Require staged upgrades to be in cros_portage_upgrade target list.

The cros_portage_upgrade script, in upgrade mode, allows for previously
staged ebuild upgrades to be present in portage-stable.  This allows for
the user to correct issues with a previously attempted upgrade.  However,
if cros_portage_upgrade is run again it must still have the staged package
upgrade as one of the upgrade targets, otherwise it will now error out.

BUG=chromium-os:21117
TEST=Run cros_portage_upgrade --upgrade with packages in sequence:
ghostscript-gpl  --> Fails because it needs poppler-data
poppler-data     --> Should now fail because ghostscript-gpl staged
                     but not requested again.
ghostscript-gpl poppler-data  --> ok

Change-Id: I80205b8d8fdbaaf1562dbcd5fbf03f8c3feef80e
Reviewed-on: http://gerrit.chromium.org/gerrit/8673
Commit-Ready: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>
Reviewed-by: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>
Tested-by: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>

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