Enable Building of packages in 3.1 branch

[Anipik]

No description provided.

[ericstj]

@joperezr this is triggering the check you added for harvesting. I think we can disable that check for this package

D:\a\1\s\.packages\microsoft.dotnet.build.tasks.packaging\1.0.0-beta.19474.3\build\Packaging.targets(425,5): error : Cannot locate package 'runtime.native.System.IO.Ports' version '4.6.0' under 'D:\a\1\s\.packages'.  Harvesting is needed to redistribute assets and ensure compatibility with the previous release.  You can disable this by setting HarvestStablePackage=false. [D:\a\1\s\pkg\runtime.native.System.IO.Ports\runtime.native.System.IO.Ports.pkgproj]
##[error].packages\microsoft.dotnet.build.tasks.packaging\1.0.0-beta.19474.3\build\Packaging.targets(425,5): error : (NETCORE_ENGINEERING_TELEMETRY=Build) Cannot locate package 'runtime.native.System.IO.Ports' version '4.6.0' under 'D:\a\1\s\.packages'.  Harvesting is needed to redistribute assets and ensure compatibility with the previous release.  You can disable this by setting HarvestStablePackage=false.
  Verifying closure of Microsoft.Private.CoreFx.NETCoreApp reference assemblies

I'd fix this by making the setting HarvestStablePackage=false in https://github.com/dotnet/corefx/blob/master/pkg/Directory.Build.props.

[ericstj]

Looks good now. Merge at will