nyan: Add a stub mainboard. BUG=None TEST=With this and other changes, built for the nyan board. BRANCH=None Change-Id: Icdde4cf5e1abb3ae1ad14279ebc129919ba30074 Signed-off-by: Gabe Black <gabeblack@google.com> Reviewed-on: https://chromium-review.googlesource.com/170837 Reviewed-by: Gabe Black <gabeblack@chromium.org> Commit-Queue: Gabe Black <gabeblack@chromium.org> Tested-by: Gabe Black <gabeblack@chromium.org>