Fix ios launchimage

Xcode throws a warning because he can't find a "Default-Portrait" launch image.
In order to respect the Cordova filename convention we should build it as follow:
<basename><usage_specific_modifiers><scale_modifier><device_modifier>.png

(Source: https://developer.apple.com/library/ios/DOCUMENTATION/iPhone/Conceptual/iPhoneOSProgrammingGuide/App-RelatedResources/App-RelatedResources.html#//apple_ref/doc/uid/TP40007072-CH6-SW12)