kbuild: centralize .dts->.dtb rule

All architectures that use cmd_dtc do so in almost the same way. Create
a central build rule to avoid duplication. The one difference is that
most current uses of dtc build $(obj)/%.dtb from $(src)/dts/%.dts rather
than building the .dtb in the same directory as the .dts file. This
difference will be eliminated arch-by-arch in future patches.

MIPS is the exception here; it already uses the exact same rule as the
new common rule, so the duplicate is removed in this patch to avoid any
conflict. arch/mips changes courtesy of Ralf Baechle.

Update Documentation/kbuild to remove the explicit call to cmd_dtc from
the example, now that the rule exists in a centralized location.

Cc: Arnd Bergmann <[e-mail]>
Cc: [e-mail]
Cc: Olof Johansson <[e-mail]>
Cc: Russell King <[e-mail]>
Acked-by: Catalin Marinas <[e-mail]>
Cc: Jonas Bonn <[e-mail]>
Cc: [e-mail]
Cc: Aurelien Jacquiot <[e-mail]>
Cc: [e-mail]
Cc: Mark Salter <[e-mail]>
Cc: Michal Simek <[e-mail]>
Cc: [e-mail]
Cc: Chris Zankel <[e-mail]>
Cc: [e-mail]
Cc: Max Filippov <[e-mail]>
Signed-off-by: Ralf Baechle <[e-mail]>
Signed-off-by: Stephen Warren <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/kbuild/makefiles.txt

···
 	in an init section in the image. Platform code *must* copy the
 	blob to non-init memory prior to calling unflatten_device_tree().
 
+	To use this command, simply add *.dtb into obj-y or targets, or make
+	some other target depend on %.dtb
+
+	A central rule exists to create $(obj)/%.dtb from $(src)/%.dts;
+	architecture Makefiles do no need to explicitly write out that rule.
+
 	Example:
-		#arch/x86/platform/ce4100/Makefile
-		clean-files := *dtb.S
-
-		DTC_FLAGS := -p 1024
-		obj-y += foo.dtb.o
-
-		$(obj)/%.dtb: $(src)/%.dts
-			$(call cmd,dtc)
+		targets += $(dtb-y)
+		clean-files += *.dtb
+		DTC_FLAGS ?= -p 1024
 
 --- 6.8 Custom kbuild commands
 

arch/mips/cavium-octeon/Makefile

···
 
 obj-y += $(patsubst %.dts, %.dtb.o, $(DTS_FILES))
 
-$(obj)/%.dtb: $(src)/%.dts FORCE
-	$(call if_changed_dep,dtc)
-
 # Let's keep the .dtb files around in case we want to look at them.
 .SECONDARY:  $(addprefix $(obj)/, $(DTB_FILES))
 

arch/mips/lantiq/dts/Makefile

···
 obj-$(CONFIG_DT_EASY50712) := easy50712.dtb.o
-
-$(obj)/%.dtb: $(obj)/%.dts
-	$(call if_changed,dtc)

arch/mips/netlogic/dts/Makefile

···
 obj-$(CONFIG_DT_XLP_EVP) := xlp_evp.dtb.o
-
-$(obj)/%.dtb: $(obj)/%.dts
-	$(call if_changed,dtc)

scripts/Makefile.lib

···
 quiet_cmd_dtc = DTC     $@
 cmd_dtc = $(objtree)/scripts/dtc/dtc -O dtb -o $@ -b 0 $(DTC_FLAGS) -d $(depfile) $<
 
+$(obj)/%.dtb: $(src)/%.dts FORCE
+	$(call if_changed_dep,dtc)
+
 # Bzip2
 # ---------------------------------------------------------------------------