fdt: Allow libfdt to be used in SPL

Add an option to enable libfdt in SPL. This can be useful when decoding
FIT files in SPL.

We need to make sure this option is not enabled in SPL by this change.
Also this option needs to be enabled in host builds. Si add a new
IMAGE_USE_LIBFDT #define which can be used in files that are built on the
host but must also build for U-Boot and SPL.

Signed-off-by: Simon Glass <sjg@chromium.org>