scripts: snippets: add DTS_EXTRA_CPPFLAGS support

Add an additional option to the `append` schema for appending to the
`DTS_EXTRA_CPPFLAGS` cmake cache variable, enabling finer control over
the content of devicetree files.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>

cmake/modules/configuration_files.cmake

@@ -13,6 +13,8 @@
 # - CONF_FILE:              List of Kconfig fragments
 # - EXTRA_CONF_FILE:        List of additional Kconfig fragments
 # - DTC_OVERLAY_FILE:       List of devicetree overlay files
+# - EXTRA_DTC_OVERLAY_FILE  List of additional devicetree overlay files
+# - DTS_EXTRA_CPPFLAGS      List of additional devicetree preprocessor defines
 # - APPLICATION_CONFIG_DIR: Root folder for application configuration
 #
 # If any of the above variables are already set when this CMake module is
@@ -118,3 +120,4 @@ zephyr_boilerplate_watch(DTC_OVERLAY_FILE)
 
 zephyr_get(EXTRA_CONF_FILE SYSBUILD LOCAL VAR EXTRA_CONF_FILE OVERLAY_CONFIG MERGE REVERSE)
 zephyr_get(EXTRA_DTC_OVERLAY_FILE SYSBUILD LOCAL MERGE REVERSE)
+zephyr_get(DTS_EXTRA_CPPFLAGS SYSBUILD LOCAL MERGE REVERSE)

doc/build/snippets/writing.rst

@@ -156,6 +156,20 @@ This :file:`snippet.yml` adds :file:`foo.conf` to the build:
 The path to :file:`foo.conf` is relative to the directory containing
 :file:`snippet.yml`.
 
+``DTS_EXTRA_CPPFLAGS``
+**********************
+
+This :file:`snippet.yml` adds ``DTS_EXTRA_CPPFLAGS`` CMake Cache variables
+to the build:
+
+.. code-block:: yaml
+
+   name: foo
+   append:
+     DTS_EXTRA_CPPFLAGS: -DMY_DTS_CONFIGURE
+
+Adding these flags enables control over the content of a devicetree file.
+
 Board-specific settings
 ***********************
 

scripts/schemas/snippet-schema.yml

@@ -13,6 +13,8 @@ schema;append-schema:
       type: str
     EXTRA_CONF_FILE:
       type: str
+    DTS_EXTRA_CPPFLAGS:
+      type: str
 
 type: map
 mapping:

scripts/snippets.py

@@ -57,6 +57,8 @@ class Snippet:
                 if not path.is_file():
                     _err(f'snippet file {pathobj}: {variable}: file not found: {path}')
                 return f'"{path}"'
+            if variable in ('DTS_EXTRA_CPPFLAGS'):
+                return f'"{value}"'
             _err(f'unknown append variable: {variable}')
 
         for variable, value in snippet_data.get('append', {}).items():