I successfully build and downloaded the u-boot 2009-08 with all Freescale patches onto the MX28EVK board via sb_loader. The final image has been build as documented with the imx-bootlets package. The same u-boot also runs fine from SD/MMC (after the correct header has been added).
Now I need to switch to a more recent version of u-boot. I build and tested u-boot from it's official git reposity. This u-boot also runs fine from SD and even (after some modification) from an SPI flash installed on my EVK. Finally I cannot download THIS u-boot with sb_loader. sd_loader stalls immediately and I do not get a single line on the debug UART.
So what might cause this issue? There must be a special difference between u-boot 2009.08 + imx-bootlets and the new u-boot (that comes with it's own SPL/preloader) that causes a USB recovery download to fail with the new version.
BTW, I can reproduce this issue als with a Linux version of sb_loader.