Makefile: Add missing CONFIG_BOARD_SIZE_LIMIT check for u-boot-spl.kwb

Currently CONFIG_BOARD_SIZE_LIMIT check is ignored for u-boot-spl.kwb
target. Fix it by adding missing $(BOARD_SIZE_CHECK) macro.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/Makefile b/Makefile
index b421296..c80b321 100644
--- a/Makefile
+++ b/Makefile
@@ -1474,6 +1474,7 @@
 
 u-boot-spl.kwb: u-boot.bin spl/u-boot-spl.bin FORCE
 	$(call if_changed,mkimage)
+	$(BOARD_SIZE_CHECK)
 
 u-boot.sha1:	u-boot.bin
 		tools/ubsha1 u-boot.bin