GitLab

This adds an MB config for 'Linux ChromeOS Buildspec Tests' builder, and makes the 'linux_packages_all' target be defined for CrOS builds as well as desktop Linux, though I have no idea if this'll actually build in that config.

TBR=dnj@chromium.org, brettw@chromium.org
BUG=643238

Review-Url: https://codereview.chromium.org/2399883003
Cr-Commit-Position: refs/heads/master@{#423739}