ubi: provide a way to skip CRC checks

Some users of static UBI volumes implement their own integrity check,
thus making the volume CRC check done at open time useless. For
instance, this is the case when one use the ubiblock + dm-verity +
squashfs combination, where dm-verity already checks integrity of the
block device but this time at the block granularity instead of verifying
the whole volume.

Skipping this test drastically improves the boot-time.

Suggested-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Richard Weinberger <richard@nod.at>