Update TFM for NETCoreApp for 1.2.0

[gkhanna79]

Update needs to be done at https://github.com/dotnet/core-setup/blob/05ce78fa94159446cb814e5fac1fa193455e7452/build_projects/dotnet-host-build/PrepareTargets.cs#L55

[weshaggard]

@ericstj I believe you have completed this work task but I will leave it to you to close.

[ericstj]

Yep. dotnet/core-setup@01157c5