make offsets in firmware update header not point to bad blocks

(This is being cherry-picked from master.)

hboot will apparently fail to install if the first block of the image
(the one pointed to by the offset in the block 0 header) is a bad
block.  (Hopefully it handles subsequent bad blocks.)

This change makes the MTD write code keep track of the bad blocks it
has skipped over, so that the offset in the header can be adjusted to
be the address of the first successfully written block.

http://b/2358012 - passion: failure to flash hboot (bad blocks?)