crosutils: comments for environment variables in make_factory_package
crosutils: comments for environment variables in make_factory_package

To help developers knowing the proper usage of such command.

BUG=none
TEST=./make_factory_package --help

Change-Id: I5a5d0efd215d0239bd56830165f0a057ad7c7474
Reviewed-on: http://gerrit.chromium.org/gerrit/9707
Reviewed-by: Tammo Spalink <130b1a4ff9567f8237a79d220d58c407e39daa31@chromium.org>
Reviewed-by: Nick Sanders <f4566bb3c90c0bf5fa0906b296b1da45051a62ee@chromium.org>
Tested-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Commit-Ready: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>

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