This is loosely based on nms-ibft-reader, but with some significant
changes. Notably, it parses /sys/firmware/ibft directly instead of
iscsiadm output.

iscsiadm is not available on early boot (perhaps it's too large) and
turns out that parsing sysfs directly is easier and more
straightforwared anyways. A win-win situation.

It is not useful alone, it's in a separate commit just for the sake of
easier review.