After trying upgrading Nuget package and then again uninstalling / reinstalling all went in vain.

Here's a workaround :-

1. Open up csproj file in edit mode
2. Add following refrences manually

<ItemGroup>

 <PackageReference Include="Microsoft.EntityFrameworkCore.Tools" Version="2.0.3" />

 <DotNetCliToolReference Include="Microsoft.EntityFrameworkCore.Tools.DotNet" Version="2.0.3" />

</ItemGroup>

Save the changes, reload the project and rebuild. You will find it working.

Now moving to command prompt, pointing to current project directory if you do a command dotnet ef and hit enter, you will start getting command options which ef core supports.

Hope it helps !