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/Makefile b/Makefile
index 42fdb73..a25845e 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2013-2019, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2013-2020, ARM Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -344,7 +344,7 @@
 endif
 
 DTC_FLAGS		+=	-I dts -O dtb
-DTC_CPPFLAGS		+=	-nostdinc -Iinclude -undef -x assembler-with-cpp
+DTC_CPPFLAGS		+=	-P -nostdinc -Iinclude -Ifdts -undef -x assembler-with-cpp
 
 ################################################################################
 # Common sources and include directories