This was tested on an 8G AC100 10V model.

The boot image tegra-2.6.29-arm2-ac100-internal-boot.img is designed to be installed
to partition 6 of the ac100 in case you want to boot from the internal eMMC.

To do that, use the normal installation image in recovery mode as described in the 
parent directory, put the unextracted tarball on your SD, format partition /dev/mmcblk0p6 as
ext3 partition (note that you wipe the old android data with this!)

Attention !! You can not repartition the mmcblk0 device, the bootloader always restores the
partition table, the boot image above uses mmcblk0p6 hardcoded to mount the rootfs.

Mount the newly formatted /dev/mmcblk0p6 under /mnt and extract the tarball with:

sudo tar xzvf tegra-rootfs.tgz --numeric-owner -C /mnt

If you reboot without recovery mode you will directly boot into the local Ubuntu install.

It is recommended to keep the SD card around so you can boot into it for recovery at any time.