kukui: Enable LTO.
kukui: Enable LTO.

Enable LTO to earn more flash spaces.
This saves 0x17cd0 - 0x1686c = 5220 bytes.

TEST=make BOARD=kukui
BUG=b:120588396
BRANCH=master

Change-Id: Id69c3f5af470d6d03170d00bc00c1e8e0699ba97
Signed-off-by: Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1663540
Commit-Queue: Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@chromium.org>
Tested-by: Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@chromium.org>
Reviewed-by: Ting Shen <c671d3efec3d7e7e285f3788bf0d1ba4c8605b78@chromium.org>
Reviewed-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Auto-Submit: Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec