cmd: bcb: introduce optional interface parameter to bcb

From: Andrii Chepurnyi <andrii_chepurnyi@epam.com>

Originally, bcb implementation relay on mmc block devices.
The interface parameter will give the ability to use bcb 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>