spl: LOAD_FIT_FULL: Relocate FDT for u-boot payloads

U-Boot expects the FDT to be located right after the _end
linker symbol (see fdtdec.c: board_fdt_blob_setup())

The "basic" LOAD_FIT path is aware of this limitation, and relocates
the FDT at the expected location. Guessing the expected location
probably only works reliably on 32-bit arm, and it feels like a hack.
One proposal would be to pass the FDT address to u-boot
(e.g. using 'r2' on arm platforms).

The variable is named "fdt_hack" to remind future contributors that,
"hey! we should fix the underlying problem". However, that is beyond
the scope of this patch.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>