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: e3fd9b53 ("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>