dm: Support legacy on disk format in dm-default-key

Version 1.0.0 did not add iv_offset to dun and did not mandate
sector size. This resulted in different on disk data format
compared to what version 2.1.0 will support. To support OTA upgrades with
legacy data format, adapt the sector size and iv_offset if legacy
encryption algorithm is used.

Change-Id: I3b7a0279bcb98c3cba9dec3f572c12d618fdc816
Signed-off-by: Neeraj Soni <neersoni@codeaurora.org>