imsm: validate multiple ppls during assemble

imsm: validate multiple ppls during assemble

Change validation algorithm to check validity of multiple ppls that
are stored in PPL area.

If read error occurs during - treat the all PPLs as invalid -
there is no guarantee that this one was not latest. If the header CRC is
incorrect - assume that there are no further PPLs in PPL area.

If whole PPL area was written at least once - there is a possibility that
old PPL (with lower generation number) will follow the recent one
(with higest generation number). Compare those generation numbers to check
which PPL is latest.

Signed-off-by: Pawel Baldysiak <pawel.baldysiak@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>