cros_workon: fix bug where cros_workon start doesn't work for first start
cros_workon: fix bug where cros_workon start doesn't work for first start

If the checkout dir doesn't already exist cros_workon start will print
the wrong dir. Fixed by using readlink -m instead.

BUG=none
TEST=Verified that first start is incorrect without this patch
but is fixed with this patch.

Change-Id: I5db29a8c1737dea1dbe4866e18576f67b9355f84

Review URL: http://codereview.chromium.org/3434008

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