Fix 2.0 chart dependency issue

[petewall]

When make build'ing the k8s-monitoring chart 2.0, it often updates every chart file.

This is because building tgz, even with the same contents, updates headers that include timestamps. That means, even if there are no changes to the feature charts, the parent chart rebundles and marks them as modified.

I'd love to make the makefile smarter to avoid changes unless the file actually has changed.