What you are seeing is the iMX6 SDP USB bootloader.
You have apparently screwed up your u-boot.
I don't own any Kindles nor do I know much about them.
I do own Glowlight Nooks (which also use iMX6SL, same as Kobo).
I use my imxboot.exe to load/experiment with secondary bootloaders.
You will need to download that at
http://www.temblast.com/android.htm
You will also need to connect up a UART to the console inside.
You will also need to get a u-boot for your Kindle.