Add nuget.config to devices directory

src/devices/nuget.config

+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+    <packageSources>
+        <clear />
+        <add key="corefxlab" value="https://dotnet.myget.org/F/dotnet-corefxlab/api/v3/index.json" />
+        <add key="NuGet.org" value="https://api.nuget.org/v3/index.json" />
+    </packageSources>
+</configuration>
\ No newline at end of file