build_packages: Switch to warn for deprecation message

BUG=b:216536410
TEST=manual (./build_packages --board ..) to verify output warning in
yellow

Change-Id: Ie5fe6b4346e164d44d6481e680ac283fa2c13003
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/3436858
Tested-by: Cindy Lin <xcl@google.com>
Auto-Submit: Cindy Lin <xcl@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Cindy Lin <xcl@google.com>