add a install_dev target

This will install the libs/headers/devkeys that people normally want.

BUG=chromium:466499
TEST=precq still passes
BRANCH=None

Change-Id: Ice035d7caddad4f05abd702d1a292b690a04d6d4
Reviewed-on: https://chromium-review.googlesource.com/259532
Trybot-Ready: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed