Add subcommand for add writing BCB only, where we provide appropriate
offsets for firmware1 and firmware2 and size.

Example of usage:
> nandbcb bcbonly 0x00180000 0x00080000 0x00200000
Writing 1024 bytes to 0x0: randomizing
OK
Writing 1024 bytes to 0x20000: randomizing
OK
Signed-off-by: NIgor Opaniuk <igor.opaniuk@toradex.com>
Tested-by: NMax Krummenacher <max.krummenacher@toradex.com>
Reviewed-by: NOleksandr Suvorov <oleksandr.suvorov@toradex.com>