scripts/dtc: only append to HOST_EXTRACFLAGS instead of overwriting

When building with

	$ HOST_EXTRACFLAGS=-g make

the expectation is that host tools are built with debug informations.
This however doesn't happen if the Makefile assigns a new value to the
HOST_EXTRACFLAGS instead of appending to it. So use += instead of := for
the first assignment.

Fixes: e3fd9b5384 ("scripts/dtc: consolidate include path options in Makefile")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Rob Herring <robh@kernel.org>

scripts/dtc/Makefile

@@ -9,7 +9,7 @@ dtc-objs	:= dtc.o flattree.o fstree.o data.o livetree.o treesource.o \
 dtc-objs	+= dtc-lexer.lex.o dtc-parser.tab.o
 
 # Source files need to get at the userspace version of libfdt_env.h to compile
-HOST_EXTRACFLAGS := -I $(srctree)/$(src)/libfdt
+HOST_EXTRACFLAGS += -I $(srctree)/$(src)/libfdt
 
 ifeq ($(shell pkg-config --exists yaml-0.1 2>/dev/null && echo yes),)
 ifneq ($(CHECK_DT_BINDING)$(CHECK_DTBS),)