Function cache_key_suffix_with_extra 

pub fn cache_key_suffix_with_extra(extra: Option<&str>) -> String

Two-segment cache key suffix accounting for an optional --extra-kconfig fragment.

The suffix uses TWO segments instead of folding both inputs into one hash:

• extra = None → kconfig_hash() only — byte-identical to cache_key_suffix, so paths that don’t expose --extra-kconfig (test / coverage / shell / verifier) keep resolving the existing keyspace and pre-1.0 cached kernels are not orphaned.
• extra = Some(content) → {kconfig_hash()}-xkc{extra_hash}, making kernel list self-describing: a reader can see at a glance which entries carry user extras and which are pure baked-in builds. Different extra content yields different xkc{...} segments, so cache discrimination across distinct --extra-kconfig invocations is structural rather than collapsed into a single opaque hash.