Use non-FHS path as default for persistent state storage

[frelon]

Right now we mount the persistent partition to /usr/local and put state-overlays/binds in /usr/local/.state.

This hides any applications installed in a derivative in /usr/local and I would propose we use some other path as default for this (/.persistent or /run/elemental/persistent comes to mind).

Users would still be able to make /usr/local into a persistent path, but our default would not mess with users expectations of a new system.

[davidcassany]

Yes we were wondering about that in the past and I think it actually makes a lot of sense. I think I'd vote to /run/elemental/persistent. This would also allow us to move as much as possible files introduced with elemental init into /etc or /usr/local, so we do not mess with distro provided stuff.