start_devserver: update relative paths in command line flags
start_devserver: update relative paths in command line flags

Because this scripts changes directory, it's possible for relative
paths to be incorrect when read by the actual dev server. This fixes
the issue by prepending the current working directory to relative path
args.

BUG=chromium-os:12122
TEST=tested relative paths w/ dev server

Change-Id: I7a85e4af5df291ec78afb0f2c1580855c38a258e

Review URL: http://codereview.chromium.org/6525011

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