I did not use fastboot on my demo K4. I modified ixtab
's method described here"
Instead of dealing with fastboot and its USB complications, I used dd to copy the main kernel to /dev/mmcblk0 with offset, as described in ixtab's README file, just before I copied the main rootfs to /dev/mmcblk0p1 with dd.
That is great for a K4 (and even a K4 demo), but on a K5 you still need fastboot to get diags+ssh on it.
I did not even use MfgTool, but instead eureka's imx_usb_downloader tool, to boot to diags.
I do not see why fastboot wouldn't work on a K4 demo, but I do not have time to try it right now. Try ixtab