skip over all-zero blocks when reading MTD partition

We fail to detect certain bad blocks (marked in the factory as bad, I
think?) when reading mtd partitions.  These come back as a block of
all zeros.  Since it's fairly unlikely a legitimate boot or recovery
block will contain 128k of zeros, change mtdutils to skip over such
blocks.

Arve says https://review.source.android.com/10535 may be a long-term
fix for this, but he isn't yet sure.