Consider supporting .d directory for base config fragments

[bgilbert]

We're starting to have divergence between the FCOS and RHCOS base configs (coreos/fedora-coreos-config#626), which requires the two distros to carry different base.ign files with common elements. Consider supporting a .d-style base config directory, where Ignition walks the directory and merges every config fragment in it.

[bgilbert]

While we're here, we might also want to support an additional .d directory for each platform ID.

[lucab]

This looks like a useful generalization of the base.ign + user.ign merge 👍.
Among other things, right now base & user configs are explicitly identified by kind. @bgilbert were you planning to have separate base.d and user.d paths, or to only touch the former, or to get rid of the split, or something else?

[bgilbert]

I was thinking this would only affect base configs for now. For user configs I think there's value in maintaining the current model, with a single well-defined root config that explicitly merges sub-configs. For base configs I think that's less important, since the "config built up from fragments" model is pretty well-understood in distro maintenance.