That flash chip is on the eInk display flex cable. On the k4nt teardown, it was dumped and contained no code. It was speculated that because is contains only "numbers" and it is attached to the replaceable display, it contains eInk calibration data.
Because the Freescale iMX50 CPU chip contains in internal flash boot ROM that is the same size as a typical uboot.bin file, it makes much more sense that u-boot is in the internal flash ROM in the CPU package.
Thanks for confirming that u-boot is not in MMC. This provides one more step on my path of discovery to writing some raw code to inject into kindle RAM and execute with USB/HID mode.