cmd: bcb: extend BCB C API to allow read/write the fields

Currently BCB C API only allows to modify 'command' BCB field.
Extend it so that we can also read and modify all the available
BCB fields (command, status, recovery, stage).

Co-developed-by: Cody Schuffelen <schuffelen@google.com>
Signed-off-by: Cody Schuffelen <schuffelen@google.com>
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>