Disable allconfigurations in corefx

There are a lot of issues to be worked through in corefx with
the allconfigurations option building on non-windows so for now
just disabling that option and will file another issut to make
that work.

Author: weshaggard

src/standard

@@ -2,7 +2,7 @@
 <Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
     <ProjectDirectory Condition="'$(PathToRepo)' != ''">$(PathToRepo)</ProjectDirectory>
-    <BuildCommand>$(ProjectDirectory)/build$(ShellExtension) -$(Configuration) -buildArch=$(Platform) -portable=false -allConfigurations -- /p:ILLinkTrimAssembly=false</BuildCommand>
+    <BuildCommand>$(ProjectDirectory)/build$(ShellExtension) -$(Configuration) -buildArch=$(Platform) -portable=false -- /p:ILLinkTrimAssembly=false</BuildCommand>
     <BuildCommand Condition="'$(Platform)' == 'arm'">$(ArmEnvironmentVariables) $(BuildCommand)</BuildCommand>
     <PackagesOutput>$(ProjectDirectory)/bin/packages/$(Configuration)</PackagesOutput>
     <CleanCommand>$(ProjectDirectory)/clean$(ShellExtension)</CleanCommand>