slippy: Initial slippy board

BUG=chrome-os-partner:19035
BRANCH=none
TEST=manual: build depthcharge for slippy board

Change-Id: Ie0346e4996e152ea862458fd99b4848ccf866688
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/49535
Reviewed-by: Aaron Durbin <adurbin@chromium.org>