cmake: modules: dts: use devicetree stub file

By providing a devicetree stub file, we make sure some internal macros
required by devicetree.h are generated in devicetree_generated.h. This
makes sure that systems without devicetree can continue working without
extra ifdeffery.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>

Author: gmarull

boards/common/stub.dts

@@ -0,0 +1,14 @@
+/*
+ * Copyright (c) 2023 Nordic Semiconductor ASA
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/* empty stub file provided for systems not using devicetree */
+
+/dts-v1/;
+
+#include <skeleton.dtsi>
+
+/ {
+
+};

cmake/modules/dts.cmake

@@ -125,10 +125,6 @@ set(DTS_CMAKE                   ${PROJECT_BINARY_DIR}/dts.cmake)
 # modules.
 set(VENDOR_PREFIXES             dts/bindings/vendor-prefixes.txt)
 
-#
-# Halt execution early if there is no devicetree.
-#
-
 # TODO: What to do about non-posix platforms where NOT CONFIG_HAS_DTS (xtensa)?
 # Drop support for NOT CONFIG_HAS_DTS perhaps?
 set_ifndef(DTS_SOURCE ${BOARD_DIR}/${BOARD}.dts)
@@ -138,10 +134,8 @@ if(EXISTS ${DTS_SOURCE})
     list(APPEND DTS_SOURCE ${BOARD_DIR}/${BOARD}_${BOARD_REVISION_STRING}.overlay)
   endif()
 else()
-  # If we don't have a devicetree after all, there's not much to do.
-  set(header_template ${ZEPHYR_BASE}/misc/generated/generated_header.template)
-  zephyr_file_copy(${header_template} ${DEVICETREE_GENERATED_H} ONLY_IF_DIFFERENT)
-  return()
+  # If we don't have a devicetree, provide an empty stub
+  set(DTS_SOURCE ${ZEPHYR_BASE}/boards/common/stub.dts)
 endif()
 
 #