of: search scripts/dtc/include-prefixes path for both CPP and DTC

Since commit d5d332d3 ("devicetree: Move include prefixes from
arch to separate directory"), cross-arch DT reference works well,
but only for CPP style #include directives.

It makes as much sense to share DT between different architectures
by using DTC's /include/ directives.

So, scripts/dtc/include-prefixes should be passed to both CPP and DTC.
I refactored Makefile.lib a bit to not repeat the same path.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>

scripts/Makefile.lib

@@ -173,8 +173,10 @@ cpp_flags      = -Wp,-MD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE)     \
 
 ld_flags       = $(LDFLAGS) $(ldflags-y)
 
+DTC_INCLUDE    := $(srctree)/scripts/dtc/include-prefixes
+
 dtc_cpp_flags  = -Wp,-MD,$(depfile).pre.tmp -nostdinc                    \
-		 -I$(srctree)/scripts/dtc/include-prefixes               \
+		 $(addprefix -I,$(DTC_INCLUDE))                          \
 		 -undef -D__DTS__
 
 # Finds the multi-part object the current object will be linked into
@@ -315,7 +317,7 @@ quiet_cmd_dtc = DTC     $@
 cmd_dtc = mkdir -p $(dir ${dtc-tmp}) ; \
 	$(CPP) $(dtc_cpp_flags) -x assembler-with-cpp -o $(dtc-tmp) $< ; \
 	$(DTC) -O dtb -o $@ -b 0 \
-		-i $(dir $<) $(DTC_FLAGS) \
+		$(addprefix -i,$(dir $<) $(DTC_INCLUDE)) $(DTC_FLAGS) \
 		-d $(depfile).dtc.tmp $(dtc-tmp) ; \
 	cat $(depfile).pre.tmp $(depfile).dtc.tmp > $(depfile)