Update parallel_emerge to restart after upgrading portage in root.
Update parallel_emerge to restart after upgrading portage in root.

We had a build failure today because portage was upgraded, and
parallel_emerge did not restart after emerging portage. This resulted
in build breakage because parallel_emerge did not pick up all of the
changes (only some of them).

BUG=chromium-os:6394
TEST=Tested that parallel_emerge restarts after upgrading portage in root,
     but not after upgrading portage in board.

Change-Id: I480dc2dc034afb5b9e0468ed6e4d16de608d3d32

Review URL: http://codereview.chromium.org/3304010

mjg59/scripts,sigma/coreos-scripts,mischief/scripts,coreos/scripts,zhang0137/scripts,sigma/coreos-scripts,endocode/scripts,glevand/coreos--scripts,smilart/scripts,bcwaldon/coreos-scripts,andrejro/coreos--scripts,zhang0137/scripts,nicescale/scripts,vmware/coreos-scripts,andrejro/coreos--scripts,gtank/scripts,marineam/coreos-scripts,nicescale/scripts,trnubo/scripts,bcwaldon/coreos-scripts,crawford/scripts,fivethreeo/scripts,mjg59/scripts,nicescale/scripts,BugRoger/scripts,bcwaldon/coreos-scripts,fivethreeo/scripts,fivethreeo/scripts,crawford/scripts,smilart/scripts,gtank/scripts,BugRoger/scripts,endocode/scripts,marineam/coreos-scripts,trnubo/scripts,mischief/scripts,sigma/coreos-scripts,smilart/scripts,mjg59/scripts,trnubo/scripts,vmware/coreos-scripts,vmware/coreos-scripts,zhang0137/scripts,crawford/scripts,coreos/scripts,marineam/coreos-scripts,andrejro/coreos--scripts,gtank/scripts,glevand/coreos--scripts,BugRoger/scripts,endocode/scripts,mischief/scripts