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>

scripts/Makefile.spl

@@ -241,8 +241,10 @@ cmd_mksunxiboot = $(objtree)/tools/mksunxiboot $< $@
 $(obj)/sunxi-spl.bin: $(obj)/$(SPL_BIN).bin FORCE
 	$(call if_changed,mksunxiboot)
 
-quiet_cmd_u-boot-spl = LD      $@
-      cmd_u-boot-spl = (cd $(obj) && $(LD) $(LDFLAGS) $(LDFLAGS_$(@F)) \
+# Rule to link u-boot-spl
+# May be overridden by arch/$(ARCH)/config.mk
+quiet_cmd_u-boot-spl ?= LD      $@
+      cmd_u-boot-spl ?= (cd $(obj) && $(LD) $(LDFLAGS) $(LDFLAGS_$(@F)) \
 		       $(patsubst $(obj)/%,%,$(u-boot-spl-init)) --start-group \
 		       $(patsubst $(obj)/%,%,$(u-boot-spl-main)) --end-group \
 		       $(PLATFORM_LIBS) -Map $(SPL_BIN).map -o $(SPL_BIN))