Trying to upgrade an IMX board to latest kernel but was looking at the ways to do it with the Legacy boot. The version of the bootloader does not include support for a separate DTB to be passed.
What you're looking for is in the kernel the ARM_APPENDED_DTB config option. This will let you include the DTB to use with the platform in the same file as the kernel. This can then be booted the normal way on your platform and the kernel will look and find the included DTB.