kbuild: cmd_dtc_cpp: extract deps from both gcc -E and dtc

Prior to this change, when compiling *.dts to *.dtb, the dependency
output from dtc would be used, and when compiling *.dtsp to *.dtb, the
dependency output from gcc -E alone would be used, despite dtc also
being invoked (on a temporary file that was guaranteed to have no
dependencies).

With this change, when compiling *.dtsp to *.dtb, the dependency files
from both gcc -E and dtc are used. This will allow cmd_dtc_cpp to
replace cmd_dtc in a future change. In turn, that will allow the C pre-
processor to be run transparently on *.dts, without the need to a
separate rule or file extension to trigger it.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>

scripts/Makefile.lib

@@ -156,7 +156,7 @@ cpp_flags      = -Wp,-MD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE)     \
 
 ld_flags       = $(LDFLAGS) $(ldflags-y)
 
-dtc_cpp_flags  = -Wp,-MD,$(depfile) -nostdinc                            \
+dtc_cpp_flags  = -Wp,-MD,$(depfile).pre -nostdinc                        \
 		 -I$(srctree)/arch/$(SRCARCH)/boot/dts                   \
 		 -I$(srctree)/arch/$(SRCARCH)/boot/dts/include           \
 		 -undef -D__DTS__
@@ -278,7 +278,8 @@ dtc-tmp = $(subst $(comma),_,$(dot-target).dts)
 
 quiet_cmd_dtc_cpp = DTC+CPP $@
 cmd_dtc_cpp = $(CPP) $(dtc_cpp_flags) -x assembler-with-cpp -o $(dtc-tmp) $< ; \
-	$(objtree)/scripts/dtc/dtc -O dtb -o $@ -b 0 $(DTC_FLAGS) $(dtc-tmp)
+	$(objtree)/scripts/dtc/dtc -O dtb -o $@ -b 0 $(DTC_FLAGS) -d $(depfile).dtc $(dtc-tmp) ; \
+	cat $(depfile).pre $(depfile).dtc > $(depfile)
 
 $(obj)/%.dtb: $(src)/%.dtsp FORCE
 	$(call if_changed_dep,dtc_cpp)