Geo,
Your uBoot envs and boot log look good so far. The one thing I would do is temporarily stop netconsole
Boot with your working SD card into Debian and
This way your UART booting will show the entire USB boot process, and hopefully we can see what went wrong.
Later, after everything is working, if you want to enable netconsole then add this uBoot env back:
fw_setenv preboot 'run preboot_nc'
Your uBoot envs and boot log look good so far. The one thing I would do is temporarily stop netconsole
Boot with your working SD card into Debian and
fw_setenv preboot fw_setenv preboot_nc 'run if_netconsole start_netconsole'
This way your UART booting will show the entire USB boot process, and hopefully we can see what went wrong.
Later, after everything is working, if you want to enable netconsole then add this uBoot env back:
fw_setenv preboot 'run preboot_nc'