xcassets: A XXxXX app icon is required for iPhone apps targeting releases of iOS prior to 7.0

When trying to handle all icons and images using xcassets I get warnings stating that 57x57, 57x57@2x, 72x72 and 72x72@2x icons are needed as I target iOS 5+ devices.

The problem is that:

• The default AppIcon group won't accept those icons.
• Adding separate entries for Icon and Icon-72 won't fix the warnings neither.