kbuild: fix a bug of the u-boot-spl link rule

cmd_u-boot-spl includes $(PLATFORM_LIBS) which changes
when CONFIG_USE_PRIVATE_GCC is updated.  The u-boot-spl image
should be re-linked if any prerequisite is newer than it
or the command line has changed.
$(call, if_changed,...) should be used instead of $(call cmd,...).
Signed-off-by: NMasahiro Yamada <yamada.m@jp.panasonic.com>

scripts/Makefile.spl

@@ -216,13 +216,13 @@ $(obj)/sunxi-spl.bin: $(obj)/$(SPL_BIN).bin
 endif
 
 quiet_cmd_u-boot-spl = LD      $@
-      cmd_u-boot-spl = cd $(obj) && $(LD) $(LDFLAGS) $(LDFLAGS_$(@F)) \
+      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)
+		       $(PLATFORM_LIBS) -Map $(SPL_BIN).map -o $(SPL_BIN))
 
-$(obj)/$(SPL_BIN): $(u-boot-spl-init) $(u-boot-spl-main) $(obj)/u-boot-spl.lds
-	$(call cmd,u-boot-spl)
+$(obj)/$(SPL_BIN): $(u-boot-spl-init) $(u-boot-spl-main) $(obj)/u-boot-spl.lds FORCE
+	$(call if_changed,u-boot-spl)
 
 $(sort $(u-boot-spl-init) $(u-boot-spl-main)): $(u-boot-spl-dirs) ;