ci: Update for buildkite environment

* Tweak west_setup:
  - log `west update` to a file to reduce noise in log
  - use `west forall` + `git reset` to make sure files are checked out
    (this is to handle a possible module cache)
* Output when we start sanity_check.  Add a banner for when we run
  sanity_check so it's a bit easier to find in console logs

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