Samus: Enable building without EC

BUG=None
TEST=Comment out EC_GOOGLE_CHROMEEC and EC_SOFTWARE_SYNC in
    src/mainboard/google/Kconfig, then build and run on Samus.

Change-Id: I973503c9c0a35ef18213aaccfa01df7eaaa5416b
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://chromium-review.googlesource.com/213799
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
3 files changed