metag: 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: Rob Herring <robh@kernel.org>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-metag@vger.kernel.org

arch/metag/Makefile

@@ -72,7 +72,7 @@ $(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/metag/boot/dts/Makefile

@@ -12,11 +12,7 @@ endif
 dtb-$(CONFIG_METAG_BUILTIN_DTB)	+= $(builtindtb-y).dtb
 obj-$(CONFIG_METAG_BUILTIN_DTB)	+= $(builtindtb-y).dtb.o
 
-targets	+= dtbs
-targets	+= $(dtb-y)
-
 .SECONDARY: $(obj)/$(builtindtb-y).dtb.S
 
-dtbs: $(addprefix $(obj)/, $(dtb-y))
-
+always += $(dtb-y)
 clean-files += *.dtb *.dtb.S