make offsets in firmware update header not point to bad blocks

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.

Change-Id: I45d58e32a36d0c1dbc0a7f871bd5985b6c8ff524
http://b/2358012 - passion: failure to flash hboot (bad blocks?)