Allow specifying the location of the main swf file inside the packaged mobile application.

By default its placed at the root of the package.
Note that this change potentially breaks builds; the original implementation put the main swf file to project.buildDir/main.swf. 
In my opinion this is not a good behavior, its not intuitive that the .swf file is placed in a subdirectory (which is named after the temporary build directory).

If the braking change a problem, I can extend this PR that keeps the old default behavior (should be just 1-2 lines)