imx: nandbcb: add support for writing BCB only

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: Igor Opaniuk <igor.opaniuk@toradex.com>
Tested-by: Max Krummenacher <max.krummenacher@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>