nand-spl: Use scripts/Makefile.build

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/Makefile b/Makefile
index b10d3b1..5204ab4 100644
--- a/Makefile
+++ b/Makefile
@@ -567,7 +567,7 @@
 		$(CPP) $(CPPFLAGS) $(LDPPFLAGS) -ansi -D__ASSEMBLY__ -P - <$< >$@
 
 nand_spl:	$(TIMESTAMP_FILE) $(VERSION_FILE) depend
-		$(MAKE) -C nand_spl/board/$(BOARDDIR) all
+		$(MAKE) $(build) nand_spl/board/$(BOARDDIR) all
 
 $(obj)u-boot-nand.bin:	nand_spl $(obj)u-boot.bin
 		cat $(obj)nand_spl/u-boot-spl-16k.bin $(obj)u-boot.bin > $(obj)u-boot-nand.bin