Handle "fix-bls" better somehow

[mvo5]

See the discussion in #248 (comment)

TL;DR: the "images" library determines (in code) right now how the "fix-bls" stage should look like, we replicate this now in otk manually but it would be nice if the equivalent of https://github.com/osbuild/images/blob/v0.90.0/pkg/manifest/os.go#L439 would be done in code. Maybe re-using the "patition" external somehow?

(see also #252)