clear BCB in misc partition before rebooting

Something is leaving behind wipe commands in the BCB area of the /misc
partition.  We don't know what is doing that.  It should always be
safe to zero out that area from uncrypt, though (because if uncrypt is
running then it's got the command we want in the recovery command file
rather than the BCB).

Bug: 16715412
Change-Id: Iad01124287f13b80ff71d6371db6371f43c43211