dtc: Remove support for common.dts

Remove the common.dts file which has been used for a year.

common.dts at one point allowed us to conditionally add an MCUBoot
overlay based on Kconfig.

but since DT lost access to Kconfig options it has been unused.

The overridable variable DTS_COMMON_OVERLAYS, which by default points
to common.dts, is also unused in-tree, and any out-of-tree usage can
be ported over to use DTC_OVERLAY_FILE instead, so we remove the
variable as well.

This simplifies the configuration system.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>

cmake/dts.cmake

@@ -17,7 +17,11 @@ set(GENERATED_DTS_BOARD_CONF      ${PROJECT_BINARY_DIR}/include/generated/genera
 set(DTS_POST_CPP                  ${PROJECT_BINARY_DIR}/${BOARD}.dts.pre.tmp)
 
 set_ifndef(DTS_SOURCE ${BOARD_DIR}/${BOARD}.dts)
-set_ifndef(DTS_COMMON_OVERLAYS ${ZEPHYR_BASE}/dts/common/common.dts)
+
+if(DEFINED DTS_COMMON_OVERLAYS)
+  # TODO: remove this warning in version 1.16
+  message(FATAL_ERROR "DTS_COMMON_OVERLAYS is no longer supported. Use DTC_OVERLAY_FILE instead.")
+endif()
 
 # 'DTS_ROOT' is a list of directories where a directory tree with DT
 # files may be found. It always includes the application directory,
@@ -31,7 +35,6 @@ list(APPEND
 
 set(dts_files
   ${DTS_SOURCE}
-  ${DTS_COMMON_OVERLAYS}
   ${shield_dts_files}
   )
 

doc/application/index.rst

@@ -1517,17 +1517,6 @@ the C preprocessor on a file which includes the following:
    can be overridden by setting the :makevar:`DTS_SOURCE` CMake
    variable.)
 
-#. Any "common" overlays provided by the build system. Currently, this
-   is just the file :file:`dts/common/common.dts`. (The common
-   overlays can be overridden by setting the
-   :makevar:`DTS_COMMON_OVERLAYS` CMake variable.)
-
-   The file :file:`common.dts` conditionally includes devicetree
-   fragments based on Kconfig settings. For example, it includes a
-   fragment for MCUboot chain-loading, located at
-   :file:`dts/common/mcuboot.overlay`, if
-   :option:`CONFIG_BOOTLOADER_MCUBOOT` is set.
-
 #. Any file or files given by the :makevar:`DTC_OVERLAY_FILE` CMake
    variable.
 

dts/common/common.dts

@@ -1,6 +0,0 @@
-/* SPDX-License-Identifier: Apache-2.0 */
-
-/*
- * Common Device Tree source, used for pulling in features and
- * additions by the build system.
- */