cros_workon: allow unmasking packages for the host
cros_workon: allow unmasking packages for the host

	modified:   cros_workon

Review URL: http://codereview.chromium.org/2945010

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