chromeos: Update .gitignore for build-related files.
chromeos: Update .gitignore for build-related files.

This makes us ignore out_* (per-board output directories,
e.g. out_x86-generic) and c (a symlink created by the Chrome
OS ebuild to limit the length of some command lines).

BUG=chromium-os:7967
TEST=ran "git status"

Review URL: http://codereview.chromium.org/5535005

git-svn-id: http://src.chromium.org/svn/trunk/src@68366 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 0fb06b9b2579710b3b97d79e063ad0bc3c132cac
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser