Update srcroot to be an absolute path.
Update srcroot to be an absolute path.

This is required so that paths still work after we change directories.
Currently our branch builder uses this broken recipe :(

BUG=none
TEST=Check that "./cros_mark_as_stable.py -r .. clean" cleans both the private overlay and chromiumos-overlay

Change-Id: Ie4047fb091bf7af36dc28482ea73b085ec12893c

Review URL: http://codereview.chromium.org/4175004

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