zinger: enable LTO

Enable Link Time Optimizations to save flash space

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>

BRANCH=none
BUG=none
TEST=run on zinger ...

Change-Id: Ie7837abd96d048b8d24dd379cf8ce4dcea1c664d
Reviewed-on: https://chromium-review.googlesource.com/271292
Reviewed-by: Alec Berg <alecaberg@chromium.org>
Commit-Queue: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>