arc: use common make variables for dtb builds

Use dtb-y and always make variables to build dtbs instead of explicit
dtbs rule. This is in preparation to support building all dtbs.
Signed-off-by: NRob Herring <robh@kernel.org>
Acked-by: NVineet Gupta <vgupta@synopsys.com>

arch/arc/Makefile

@@ -121,7 +121,7 @@ $(boot_targets): vmlinux
 	$(Q)$(MAKE) $(build)=$(boot)/dts $(boot)/dts/$@
 
 dtbs: scripts
-	$(Q)$(MAKE) $(build)=$(boot)/dts dtbs
+	$(Q)$(MAKE) $(build)=$(boot)/dts
 
 archclean:
 	$(Q)$(MAKE) $(clean)=$(boot)

arch/arc/boot/dts/Makefile

@@ -6,10 +6,10 @@ ifneq ($(CONFIG_ARC_BUILTIN_DTB_NAME),"")
 endif
 
 obj-y   += $(builtindtb-y).dtb.o
-targets += $(builtindtb-y).dtb
+dtb-y := $(builtindtb-y).dtb
 
 .SECONDARY: $(obj)/$(builtindtb-y).dtb.S
 
-dtbs:  $(addprefix  $(obj)/, $(builtindtb-y).dtb)
 
+always := $(dtb-y)
 clean-files := *.dtb  *.dtb.S