[build] Added `buildall` target
[build] Added `buildall` target

This target will attempt to build all relevant targets (so, targets can run on the current platform) for every item in the manifest file within NUnit.  This will help build every target without stopping on the first failure, and should also provide us with formatted nunit results output so we can track failing projects more easily.

bholmes/XamarinComponents,xamarin/XamarinComponents,bholmes/XamarinComponents,xamarin/XamarinComponents,bholmes/XamarinComponents,bholmes/XamarinComponents,xamarin/XamarinComponents,xamarin/XamarinComponents,xamarin/XamarinComponents,bholmes/XamarinComponents,xamarin/XamarinComponents,bholmes/XamarinComponents