Tegra2 SoC has 4 standard serial ports available. One of them is often used as the Console port both for the Fastboot Android Bootloader and for the kernel console. This port is not directly useable on the AC100. it's available on the mother board on a small connector (labelled JP1). The connector is not soldered on Production devices.

This page explains how to add serial port to an USB port on your host PC.