ci: Add initial buildkite ci setup

Add setup to utilize buildkite for CI purposes:

1. .buildkite/hooks/pre-command:
   * Handles getting git checkout setup against upstream repo
   * Setup some west module cache (dirs, clean out files & locks)
   * init dir for ccache

2. .buildkite/hooks/post-command:
   * Report disk usage (meant for possible debugging)

3. .buildkite/pipeline.yml [uses to determine what to do]:
   * setup zephyr env vars
   * set which docker container to use
     (export some local disk caches for git, west modules, and ccache)
   * uses plug to general build annotation on failure (junit-annotate)

4. .buildkite/run.sh [ buildkite wrapper to invoke scripts/ci/run.sh ]

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>