image: Support FDTs already loaded at their load address

boot_get_fdt() expects a uImage-wrapped FDT to be loaded to a staging
location, and then memmove()s it to the load address specified in the
header. This change enhances boot_get_fdt() to detect when the image has
already been loaded to the correct address, and skip this memmove(). The
detection algorithm was written to match the equivalent for the kernel;
see bootm_load_os()'s IH_COMP_NONE case.

v2: New patch

Signed-off-by: Stephen Warren <swarren@nvidia.com>