cmd: avb: introduce optional interface parameter to avb init

From: Andrii Chepurnyi <andrii_chepurnyi@epam.com>

Originally, avb implementation relay on mmc block devices.
The interface parameter will give the ability to use avb with
various block devices by choosing the exact interface type.
By default (if no interface parameter is provided) mmc interface
will be used.

Reviewed-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Signed-off-by: Andrii Chepurnyi <andrii_chepurnyi@epam.com>