bootm: Fix upper bound of FDT overlap checks

FTD blob can be put immediately after the OS image.
So use strict inequality for start address check.

Fixes: fbde7589 ("common: bootm: add checks to verify if ramdisk / fdtimage overlaps OS image")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>