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:

<pre>basename + usage_specific_modifiers + scale_modifier + device_modifier + .png</pre>

(Source: https://developer.apple.com/library/ios/DOCUMENTATION/iPhone/Conceptual/iPhoneOSProgrammingGuide/App-RelatedResources/App-RelatedResources.html#//apple_ref/doc/uid/TP40007072-CH6-SW12)