use liboverdrop crate

[rfairley]

Use the API provided by coreos/liboverdrop-rs#1.

[rfairley]

Accidentally pushed 63f9036 to master, closing this PR. This was a simple change to adapt to the changes in the liboverdrop master - so I think it is OK not to revert it. A follow-up can be done soon to use the liboverdrop crate once published.

[lucab]

I guess this means we now need to release and fedora-package liboverdrop before being able to release and package this. Timewise, do you think it's fine?

[rfairley]

Right, yes this does make packaging liboverdrop a blocker. The packaging for liboverdrop should be straightforward, but I don't want to rush work there for this. I'll revert this now - less harm done.

I did build an RPM ready go here for FCOS, but we may still want to add changes upstream (in this repo) before packaging this. Shouldn't block more changes being added based on my accidental push.

[rfairley]

Superseded by: #10