scripts: Makefile: add support to use external DTC

Add support to use external DTC to compile device tree blobs.

Change-Id: I268d2332d5328c3b0050b35c96f91e8ef9e1f6f8
Signed-off-by: Shashank Mittal <mittals@codeaurora.org>

scripts/Makefile.lib

@@ -261,10 +261,16 @@ cmd_dt_S_dtb=						\
 $(obj)/%.dtb.S: $(obj)/%.dtb
 	$(call cmd,dt_S_dtb)
 
+ifneq ($(DTC_EXT),)
+DTC = $(DTC_EXT)
+else
+DTC = $(objtree)/scripts/dtc/dtc
+endif
+
 quiet_cmd_dtc = DTC     $@
 cmd_dtc = mkdir -p $(dir ${dtc-tmp}) ; \
 	$(CPP) $(dtc_cpp_flags) -x assembler-with-cpp -o $(dtc-tmp) $< ; \
-	$(objtree)/scripts/dtc/dtc -O dtb -o $@ -b 0 \
+	$(DTC) -O dtb -o $@ -b 0 \
 		-i $(dir $<) $(DTC_FLAGS) \
 		-d $(depfile).dtc.tmp $(dtc-tmp) ; \
 	cat $(depfile).pre.tmp $(depfile).dtc.tmp > $(depfile)

scripts/dtc/Makefile

@@ -1,7 +1,9 @@
 # scripts/dtc makefile
 
 hostprogs-y	:= dtc
+ifeq ($(DTC_EXT),)
 always		:= $(hostprogs-y)
+endif
 
 dtc-objs	:= dtc.o flattree.o fstree.o data.o livetree.o treesource.o \
 		   srcpos.o checks.o util.o