Add CONFIG_I2C_SLAVE_ONLY for boards that only operate as a slave
on i2c.
BUG=chrome-os-partner:41959
BRANCH=none
TEST=make BOARD=glados_pd and see 2kB flash savings
Change-Id: I30831ce48b391d985c25e266229d5c6f2312042b
Signed-off-by: Alec Berg <alecaberg@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/306783
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>