Migrations in EF Core RC2

When I try to add migration I get:

This preview of Entity Framework tools does not support targeting class library projects in ASP.NET Core and .NET Core applications. See http://go.microsoft.com/fwlink/?LinkId=798221 for details and workarounds.

But in my project.json file I have this:

"frameworks": {
    "net461": {
       "buildOptions": {
          "emitEntryPoint": true
       }
    }
}

What can I do?

Thanks!