dev: Add .gitignore file.
dev: Add .gitignore file.

BUG=none
TEST=none
TBR=anush

Review URL: http://codereview.chromium.org/2823012

marineam/coreos-dev-util,coreos/dev-util,mgireesh05/dev-util,mgireesh05/dev-util,mgireesh05/dev-util,coreos/dev-util,marineam/coreos-dev-util,coreos/dev-util