Make the Framework WinFX targets import opt-out

Currently there's no way to properly switch between NETFX's and CoreCLR's WinFX targets.
This adds an opt-out, just in case, if we want to use the NETFX's WinFX targets instead.

packaging/Microsoft.NET.Sdk.WindowsDesktop/targets/Microsoft.NET.Sdk.WindowsDesktop.props

@@ -165,9 +165,10 @@
     Workaround: https://github.com/microsoft/msbuild/issues/4948
     Disable .NET Framework's inbox WinFX targets when using the SDK, since, we really don't use it's build logic
     and is superseded by 'WindowsDesktop' SDK that provides it's own WinFX for both NETFX and CoreCLR targets.
+    Make it opt-out, just in case, if something fails or we don't want to use 'WindowsDesktop' SDK's version.
   -->
   <PropertyGroup>
-    <ImportFrameworkWinFXTargets>false</ImportFrameworkWinFXTargets>
+    <ImportFrameworkWinFXTargets Condition="'$(ImportFrameworkWinFXTargets)' == ''">false</ImportFrameworkWinFXTargets>
   </PropertyGroup>
 
 </Project>