how to build eclipse rcp application from .product with ant on jenkins
If you really want ant, check PDE build.
However, I really recommend using Tycho instead. It makes building an Eclipse RCP a lot easier.
I've been down this road you are on. Trust me... you will end up with something incredibly more complicated with Ant than you would if you just use Tycho/Maven. You said you want it simple as possible, and IMHO, Tycho is it.
There are examples here... http://wiki.eclipse.org/Tycho/Packaging_Types
I also suggest to check out tycho from git and look around the tycho-demo location. They have great examples there too.
http://git.eclipse.org/c/tycho/org.eclipse.tycho.git/tree/tycho-demo/itp04-rcp
Plus, Jenkins is super easy when dealing with Maven builds.
The current version of Tycho is 0.13.0. It's pretty early in it's life, and under active development, so it's getting better every day.