I've repaired my install by manually applying the failed dd step.
1. Mount the SD card on another Linux machine and open up a terminal.
2. `cd` to the SD card, for me it's `cd /media/scott/sdcard/`
3. `cd` to the u-boot image directory `cd usr/lib/linux-u-boot-current-tinkerboard/`
4. make note of which device the sd card is, you could use `mount | grep media` for example
5. for me it's `/dev/sdb` but it could be something like `/dev/mmcblk1`
6. IMPORTANT make sure you absolutely have the right device
7. run the following, replacing XXX with your target device `dd if=u-boot-rockchip-with-spl.bin of=/dev/XXX bs=32k seek=1 conv=notrunc`