Simplify boot storage

it returns list of pairs, each pair represent udev and its kernel_device. so first first mean use first available pair and use udev device from it. Of course it is possible to use some kind of openstruct, but that way it will be hard to create hash above, so in the end I will do two translations.
My primary goal is to create easy to use class. Internals is just second class citizen as long as it is properly tested.