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.
(cherry picked from commit 1db54a5522a2d523e406ce8713bfe88bb9e3f657)
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.