As far as I know there are no debuggers embedded in the firmware and no one has published a way to connect to JTAG port yet.
I'm sure it's possible to create our own debugger which would be embedded into the firmware and which would communicate to the PC through the firmware's USB functions (or, even better, using a serial port). It would setup breakpoints and simulate single stepping by patching the firmware code, as ARM debuggers normally do.
But it's 1) waaaaay too complicated even for me 2) just a waste of time, to be honest.