snippets: support for EXTRA_DTC_OVERLAY_FILE and EXTRA_CONF_FILE

This commit adds change snippets to use the new EXTRA_DTC_OVERLAY_FILE
and EXTRA_CONF_FILE Zephyr build system variables instead of
DTC_OVERLAY_FILE and OVERLAY_CONFIG.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
diff --git a/scripts/snippets.py b/scripts/snippets.py
index 2158613..74e7ebb 100644
--- a/scripts/snippets.py
+++ b/scripts/snippets.py
@@ -52,7 +52,7 @@
         '''Process the data in a snippet.yml file, after it is loaded into a
         python object and validated by pykwalify.'''
         def append_value(variable, value):
-            if variable in ('DTC_OVERLAY_FILE', 'OVERLAY_CONFIG'):
+            if variable in ('EXTRA_DTC_OVERLAY_FILE', 'EXTRA_CONF_FILE'):
                 path = pathobj.parent / value
                 if not path.is_file():
                     _err(f'snippet file {pathobj}: {variable}: file not found: {path}')