BLS: Specification Conformance

The BLS support in Sprout is currently rudimentary. We should closely follow the BLS specification, including any deviations from the spec that are used in practice.

Possible deviations include the behavior of tuned on Fedora, which in fc42 adds $tuned_initrd to the initrd lines.
Also we don't merge initrds if there are multiple, this should be fixed.