Add optional logging prefix to info, warn and error messages.
Add optional logging prefix to info, warn and error messages.

BUG=chromium-os:11981
TEST=run cros_sign_bootstub with and without CROS_LOG_PREFIX set.

Change-Id: I320622d4161479dcc29ebdee68e160d94f1d3196

Review URL: http://codereview.chromium.org/6597053

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