kbuild: add dtbs_prepare target

Factor out the common prerequisites for DT compilation into the new
target, dtbs_prepare.

Add comments to explain why include/config/kernel.release is the
prerequisite. Our policy is that installation targets must not rebuild
anything in the tree. If 'make modules_install' is executed as root,
include/config/kernel.release may be owned by root.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Tested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>

Makefile

@@ -1370,16 +1370,21 @@ endif
 
 ifneq ($(dtstree),)
 
-%.dtb: include/config/kernel.release scripts_dtc
+%.dtb: dtbs_prepare
 	$(Q)$(MAKE) $(build)=$(dtstree) $(dtstree)/$@
 
-%.dtbo: include/config/kernel.release scripts_dtc
+%.dtbo: dtbs_prepare
 	$(Q)$(MAKE) $(build)=$(dtstree) $(dtstree)/$@
 
-PHONY += dtbs dtbs_install dtbs_check
-dtbs: include/config/kernel.release scripts_dtc
+PHONY += dtbs dtbs_prepare dtbs_install dtbs_check
+dtbs: dtbs_prepare
 	$(Q)$(MAKE) $(build)=$(dtstree)
 
+# include/config/kernel.release is actually needed when installing DTBs because
+# INSTALL_DTBS_PATH contains $(KERNELRELEASE). However, we do not want to make
+# dtbs_install depend on it as dtbs_install may run as root.
+dtbs_prepare: include/config/kernel.release scripts_dtc
+
 ifneq ($(filter dtbs_check, $(MAKECMDGOALS)),)
 export CHECK_DTBS=y
 dtbs: dt_binding_check