I've already done it, but with dtb and kernel files instead of itb:
(I'm using OpenWRT distribution instead of Yocto)
Two partitions in the usb drive, made with gparted in linux.
First one, a small FAT partition, second an ext4 file system.
In the first one, I've put the following files:
image-fsl-ls1012a-frdm.dtb
ls1012afrdm-kernel.bin
In the second partition, the linux file system.
Uboot commands:
usb start
fatload usb 0:1 90000000 image-fsl-ls1012a-frdm.dtb
fatload usb 0:1 96000000 ls1012afrdm-kernel.bin
setenv bootargs "console=${console},${baudrate} root=/dev/sda2 rootwait rw"
bootm 96000000 - 90000000
In your case, this should work, if the itb file is in root of usb drive:
usb start
ext2load usb 0:1 90000000 kernel-fsl-ls1012a-frdm.itb
setenv bootargs "console=${console},${baudrate} root=/dev/sda1 rootwait rw"
bootm 90000000
Hope this helps.