Replace dts includes with C preprocessor syntax

Using the /include/ syntax, the include was evaluated by dtc, only after running
the preprocessor, therefore the .dtsi files were not preprocessed. This patch
adds the #include syntax instead. Evaluating this and preprocessing the files
now happens in a single step, done by the C preprocessor.

Change-Id: I6d0104b6274316fc736e84973502a4d6c2c9d6e0
Signed-off-by: Balint Dobszay <balint.dobszay@arm.com>
diff --git a/make_helpers/build_macros.mk b/make_helpers/build_macros.mk
index 47f3ebd..6b72cfd 100644
--- a/make_helpers/build_macros.mk
+++ b/make_helpers/build_macros.mk
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2015-2019, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2015-2020, ARM Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -513,7 +513,7 @@
 	$(eval DTBS       := $(addprefix $(1)/,$(call SOURCES_TO_DTBS,$(2))))
 	$$(Q)$$(PP) $$(DTC_CPPFLAGS) -MT $(DTBS) -MMD -MF $(DTSDEP) -o $(DPRE) $$<
 	$${ECHO} "  DTC     $$<"
-	$$(Q)$$(DTC) $$(DTC_FLAGS) -i fdts -d $(DTBDEP) -o $$@ $(DPRE)
+	$$(Q)$$(DTC) $$(DTC_FLAGS) -d $(DTBDEP) -o $$@ $(DPRE)
 
 -include $(DTBDEP)
 -include $(DTSDEP)