carvel-ytt#769 recommended

config.yaml

#@ load("@ytt:data", "data")
#@ load("@ytt:yaml", "yaml")

#@ def config():
cluster: #@ data.values.cluster.config
#@ end

---
apiVersion: v1
data:
  config.yaml: #@ yaml.encode(config())
kind: ConfigMap
metadata:
  name: my-cm
  namespace: default

values.yaml

#@data/values
---
cluster:
  config: val