Support `{r,b}oot-mount-spec` and `stateroot` in install config

[jbtrystram]

Let's make the --boot-mount-spec and --root-mount-spec and --stateroot options of bootc install supported by the install config file

See osbuild/bootc-image-builder#1180

This is helping for coreos/fedora-coreos-tracker#1827

[cgwalters]

@jbtrystram were you planning to pick this up? When filing issues like this can you also just for procedural clarity cross-link it to the larger tracker work that (I believe) you're aiming for of using bootc install for coreos?

[jbtrystram]

@jbtrystram were you planning to pick this up?

Yes, already have some code, I'll just do a bit more testing before opening a PR.