Add Microsoft.NETCore.Platforms to the vertical build

Author: weshaggard

pkg/Microsoft.NETCore.Platforms/Configurations.props

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <PropertyGroup>
+    <BuildConfigurations>
+      netstandard;
+    </BuildConfigurations>
+  </PropertyGroup>
+</Project>

pkg/Microsoft.NETCore.Platforms/Microsoft.NETCore.Platforms.builds

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.props))\dir.props" />
-  <ItemGroup>
+  <ItemGroup Condition="'$(OfficialBuildId)' == '' Or '$(BuildAllConfigurations)' == 'true'">
     <Project Include="Microsoft.NETCore.Platforms.pkgproj" />
   </ItemGroup>
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.traversal.targets))\dir.traversal.targets" />