Add --regulatory-label-dir flag

This adds a flag to overwrite the default directory of regulatory
labels (i.e. /usr/share/chromeos-assets/regulatory_labels).
It's mainly to support model-specific regulatory labels.

Bug: 921914
Change-Id: Ia6ffc5fc8741eab757cc6dcbcc9536e8f54add9c
Reviewed-on: https://chromium-review.googlesource.com/c/1461835
Commit-Queue: Philip Chen <philipchen@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Auto-Submit: Philip Chen <philipchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631581}
3 files changed