Different boards using u-boot SPL require to write to different
locations. Sometimes, the 8MiB gap isn't sufficient - rk3399 boards
write to 0x16384 for example, which is at 8MiB, thus overriding the
fat32 partition with the SPL.
This is supeer useful to allow the normal sd-image code to be used by
someone who wants to setup multiple partitions with a sd-image.
Currently I'm manually copying the sd-image file and modifying it
instead.