scripts: Add helper script to generate eMMC block device images

As an eMMC block device image may consist of more than just the user
data partition, provide a helper script that can compose the image from
boot partitions, an RPMB partition and the user data image. The script
also does the required size validation and/or rounding.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <eecefa8e-44ae-45ff-85d0-3f9b786948e0@siemens.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>