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>
diff --git a/scripts/snippets.py b/scripts/snippets.py
index 878c0c2..9662f3e 100644
--- a/scripts/snippets.py
+++ b/scripts/snippets.py
@@ -57,6 +57,8 @@
                 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():