Move sanity checks for missing directories until after checkout happens.
Move sanity checks for missing directories until after checkout happens.

Preflight clean needs to wait until after the checkout happens before
it asserts that all of our overlays exist.

BUG=chromium-os:9197
TEST=Ran cbuildbot.py with missing chromiumos-overlay dir

Change-Id: If66574f01d1e85741e971919a9fa2da34f85872d

Review URL: http://codereview.chromium.org/5069001

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