mmc: Enable support for Triangle Away

Exposes /dev/block/mmcblk0boot0 and /dev/block/mmcblk0boot1, where
the custom binary flash count is stored.  Re-exposing these enables
the use of utilities like Triangle Away to reset the counter.

Change-Id: I4a6d4784075b4f993c7e0be4a4b7890356104ca5