build: separate preprocessing from DTB compilation

This is a small change to separate preprocessing of device tree sources
into their own build step as opposed to combining them. The impact of
this is more on separation of concerns than anything tangible, but it's
helpful to avoid doing more than necessary in a build rule.

Change-Id: I770291bd9d9f627c93e82556a40f753bf27eef93
Signed-off-by: Chris Kay <chris.kay@arm.com>

make_helpers/build_macros.mk

@@ -644,12 +644,14 @@ $(eval DTSDEP := $(patsubst %.dtb,%.o.d,$(DOBJ)))
 # Dependencies of the DT compilation on its pre-compiled DTS
 $(eval DTBDEP := $(patsubst %.dtb,%.d,$(DOBJ)))
 
-$(DOBJ): $(2) $(filter-out %.d,$(MAKEFILE_LIST)) | fdt_dirs
+$(DPRE): $(2) | fdt_dirs
 	$${ECHO} "  CPP     $$<"
 	$(eval DTBS       := $(addprefix $(1)/,$(call SOURCES_TO_DTBS,$(2))))
 	$$(Q)$($(ARCH)-cpp) -E $$(TF_CFLAGS_$(ARCH)) $$(DTC_CPPFLAGS) -MT $(DTBS) -MMD -MF $(DTSDEP) -o $(DPRE) $$<
+
+$(DOBJ): $(DPRE) $(filter-out %.d,$(MAKEFILE_LIST)) | fdt_dirs
 	$${ECHO} "  DTC     $$<"
-	$$(Q)$($(ARCH)-dtc) $$(DTC_FLAGS) -d $(DTBDEP) -o $$@ $(DPRE)
+	$$(Q)$($(ARCH)-dtc) $$(DTC_FLAGS) -d $(DTBDEP) -o $$@ $$<
 
 -include $(DTBDEP)
 -include $(DTSDEP)