Hmm. OK looking at uboot variables this is what it seems to be doing. First several environment variables are used:
$this->bbcode_second_pass_code('', 'memoffset_kernel=0x02000000
memoffset_initrd=0x08004000
bootargs_console=console=ttyS0,115200
bootargs_mtdparts=mtdparts=orion_nand:640k(u-boot)ro,16k(u-boot-env),-(iomega-firmware)ro
bootargs_root=root=LABEL=ROOT rw
')
Then usb_load is run:
$this->bbcode_second_pass_code('', 'setenv bootargs ${bootargs_console} ${bootargs_mtdparts} ${bootargs_root};
usb reset;
ext2load usb 0:1 ${memoffset_kernel} /uImage;
ext2load usb 0:1 ${memoffset_initrd} /uInitrd;
bootm ${memoffset_kernel} ${memoffset_initrd};')
Then sata_load is run:
$this->bbcode_second_pass_code('', 'setenv bootargs ${bootargs_console} ${bootargs_mtdparts} ${bootargs_root};
ide reset;
ext2load ide 0:1 ${memoffset_kernel} /uImage;
ext2load ide 0:1 ${memoffset_initrd} /uInitrd;
ext2load ide 1:1 ${memoffset_kernel} /uImage;
ext2load ide 1:1 ${memoffset_initrd} /uInitrd;
bootm ${memoffset_kernel} ${memoffset_initrd};')
To my mind what to note is that usb_load is always run first, and it has no tests for bits failing, so I guess it always runs the $this->bbcode_second_pass_code('', 'bootm ${memoffset_kernel} ${memoffset_initrd};') at the end.
Now look at you boot, it scans USB as told, then it doesn't find any devices, then it runs the boot. But as it hasn't found any usb devices, it hasn't loaded the uImage or the uInitrd, it fails during the boot.
So looks like your boot process is wrong, it never gets to the sata_load.
But whats odd, is bootm is saying it has found uimages at the locations in memory passed to it.
I'd be tempted to type commands by hand and see what is needed, then adapt the uboot env as needed. Couldn't see the offset error you said the device tree kernel image had, but yet its the kind of think to check. Pity you can't do the modern way of passing the device tree (as third argument to boot), so have to use the hack of adding it to the end of the kernel image.
Have to wonder if anyone has done a modern uboot for the box, as you have the uart connected my be tempted to do the kwboot across the uart, to try more modern uboot. Only do this is you know what you are doing though ...