mb/google/kahlee: Remove mainboard.h

mainboard.h only had a single function definition.  Move it into
baseboard/variants.h and get rid of the file.

TEST=Build grunt/kahlee
BUG=b:69164070

Change-Id: I6b7d50d5c949733d77c42b4daf56ed1f97ed6954
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/22434
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
5 files changed