ci/build_system_check: improve check for features provided in Makefile.features

- Take into account potential spaces at the beginning of a line
- Check all Makefile.<something> files, not only the ones in boards and cpus

dist/tools/buildsystem_sanity_check/check.sh

@@ -66,14 +66,14 @@ check_providing_features_only_makefile_features() {
     local patterns=()
     local pathspec=()
 
-    patterns+=(-e 'FEATURES_PROVIDED *+= *')
+    patterns+=(-e '^[ ]*FEATURES_PROVIDED *+= *')
 
-    pathspec+=("boards/*Makefile*" "cpu/*Makefile*")
+    pathspec+=("*Makefile\.*")
 
     pathspec+=(":!*Makefile.features")
 
     git -C "${RIOTBASE}" grep "${patterns[@]}" -- "${pathspec[@]}" \
-        | error_with_message 'Features in cpu and boards should only be provided in Makefile.features files'
+        | error_with_message 'Features should only be provided in Makefile.features files'
 }
 
 # Some variables do not need to be exported and even cause issues when being