deploy: Set the immutable bit on the deployment root

This prevents people from creating new directories there and expecting
them to be persisted.  The OSTree model has all local state to be in
/etc and /var.

This introduces a compile-time dependency on libe2fsprogs.

We're only doing this for the root directory at the moment.

https://bugzilla.gnome.org/show_bug.cgi?id=728006