Use a single base www directory as the source for multiple platforms?

Use a single base www directory as the source for multiple platforms?

I understand that by doing :
phonegap build android
a copy of the files in www is made in platforms/android/assets/www
I can then import the platforms/android folder in my IDE as a Android
project and run it from there in an emulator or my phone.
But I would like to have only 1 code base that I can use to build for
different platforms, so ideally I would like to be able to make changes to
the app in the www in the root only.
I guess I need some kind of file-copying builder, that is started before
running the project, but I have no experience with this. I read about Ant,
Maven and Gradle....is this the way to go ?
Could anybody tell me how everybody is doing this? Or is it really not
common to use 1 single code base for all platforms? (I thought that was
the whole point of Phonegap!)