At the moment there is no way you can use pure PowerShell to do this because a utility like migrate.exe does not exist yet and importing the EF PS modules is not possible as they require a Package Manager PowerShell Host.
Here are some ideas how you can update a remote db in EF7:
One thing you could do is use the package manager console commands from within VS as usual to update the remote db. You can create a second context that has the remote db connection string and use the update-database command specifying the context to use. These commands require the following package in EF7:
https://www.nuget.org/packages/EntityFramework.Commands/.
I have done this successfully in a class lib project.
Another solution would be to use DNX commands by creating a DNX project instead of a classic one. DNX projects are not just for web sites, it is just another type of project. Here is a link that shows how to create a console app DNX project:
http://docs.asp.net/en/latest/dnx/console.html.
So with this type of project you can use the provided DNX commands that you seem to be aware of.
I hope this helped. Maybe we can give more help if you describe your situation and your end goal in more detail.