How do I enable USB tethering on a JellyBean device? Does JellyBean support USB tether?

Is there a way compile the JB source so USB tether is always enabled?
I would like to have the support through the OS and not depend on an App.
Yes.

On ICS the setting can be found at

Settings -&gt; Wireless &amp; Networks -&gt; More.. -&gt; Tethering &amp; portable hotspot -&gt; USB tethering

I think it should be at the same place on Jelly Bean. Note that you need a device with a cellular data chip for this feature. Devices without (e.g. the Nexus 7) won't show this option.

USB tethering support was added with the 2.2 (Froyo) release