Transpile merged butane configs into ignition

[strass]

I have separated my butane config into logical units and they are bundled back using config merging. I assumed that referenced butane configs would be stored as ignition files, but it looks like they are just the same butane files in gzip'd base64. I think that they should be converted upon read to remove this footgun.

[bgilbert]

Thanks for the report. This is a duplicate of #118.