mtd: bcm63xxpart: check the image tag's crc32

Only use the values from the image tag if it is valid. Always create
the CFE, NVRAM and linux partitions, to allow flashing a new image even
if the old is invalid without overwriting CFE or NVRAM.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>