ENYO-3343: Allow the selection of a target platform to build a Phonegap application.

Link to the JIRA ticket: https://enyojs.atlassian.net/browse/ENYO-3343

Ready for review.

Tested on Chrome, FireFox, (Windows 7)
Enyo-DCO-1.1-Signed-off-by: Rafik Adiche mahmoud-rafik.adiche@hp.com