hw/sd/sdcard: Refactor sd_bootpart_offset

hw/sd/sdcard: Refactor sd_bootpart_offset

This function provides the offset for any partition in the block image,
not only the boot partitions, therefore rename it. Align the constant
names with the numbering scheme in the standard and use constants for
both boot partitions for consistency reasons. There is also no reason to
return early if boot_part_size is zero because the existing code will
provide the right value in that case as well.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <66e9b07476aad61820c4f42f4f984cc90752ba5e.1756706188.git.jan.kiszka@siemens.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>