cmd: bcb: support various block device interfaces for BCB command

Currently BCB command-line, C APIs and implementation only
support MMC interface. Extend it to allow various block
device interfaces.

Signed-off-by: Dmitrii Merkurev <dimorinny@google.com>
Cc: Eugeniu Rosca <erosca@de.adit-jv.com>
Cc: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Cc: Sean Anderson <sean.anderson@seco.com>
Cc: Cody Schuffelen <schuffelen@google.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com> # on vim3
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>