dts, api, test: convert makefiles to Kbuild style

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/Makefile b/Makefile
index 03389dd..7b05b53 100644
--- a/Makefile
+++ b/Makefile
@@ -383,7 +383,7 @@
 all:		$(ALL-y) $(SUBDIR_EXAMPLES)
 
 $(obj)u-boot.dtb:	checkdtc $(obj)u-boot
-		$(MAKE) -C dts binary
+		$(MAKE) $(build) dts binary
 		mv $(obj)dts/dt.dtb $@
 
 $(obj)u-boot-dtb.bin:	$(obj)u-boot.bin $(obj)u-boot.dtb