dm verity: Add an optional arg to wait for target devices

Add an optional arg (device_wait) to allow indefinite
wait for target dm devices. This could be required for
devices which come up asynchronously. As existing
implementation had some of the optional args dependent
on processing of required args, introduce a new function
to allow optional args pre processing, for args which
affect the processing of required args.

Change-Id: I61af9a6136cefaf5e20cc08c0c891d838ad75b7f
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>