uboot-imx

Makefile: Allow the SPL final link rule to be overridden

Overriding the final link rule is possible with U-Boot proper. It us used to
create a sandbox image links with host libraries. To build a sandbox SPL
image we need the same feature for SPL.

To support this, update the SPL link rule so sandbox can override it.

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