Support sfdisk backend for partitioning

sfdisk is part of util-linux and has supported GPT partitioning for a while now. It'd be desirable to support using sfdisk instead of sgdisk for partitioning for OSes that want to shed that dependency.

Proposal:

1. add a new sfdisk package that implements the same API as the sgdisk one
2. support a compile option to change the default backend to use from sgdisk to sfdisk
3. support a kernel argument option (e.g. ignition.partition-backend=(sfdisk|sgdisk)) to override compile-time defaults
4. once we have some confidence in the code, change the upstream default to sfdisk and declare the sgdisk backend deprecated
5. after a reasonable window, drop support for the sgdisk backend

On the FCOS side, we can start using the sfdisk backend in some of the non-production streams to get some early testing, and if deemed necessary, bake it in the next stream as well.