* Make sure rid specific libs projs get packaged

RID specific runtime.*.runtime.native.System.IO.Ports projects weren't
packaged because the GeneratePackageOnBuild property wasn't set to true
for them. The property was only true during an allconfigurations build.
As these projects are only built outside of an allconfigurations build,
the GeneratePackageOnBuild property needs to account for such as well.

Also updating the NoTargets Sdk to clean the rid specific package up.

Moving the GeneratePackageOnBuild logic into a props file and set it to
false during servicing (or runtimelab) so that devs can set the property
during servicing directly in the project file if a project should be
packaged.

* Remove the GeneratePackage property

That property was intended to enabled incremental servicing but with
now using the NuGet Pack task we can just make use of the publicl
available GeneratePackageOnBuild property.

* Fix IsPackable setting