From the mx6 reference manual:
"The i.MX 6Series boot ROM supports the following boot devices:
NOR Flash
NAND Flash
OneNAND Flash
SD/MMC
Serial ATA (SATA) HDD
Serial ROM devices"
,so you you cannot boot directly from a USB thumb.
What you can do is to boot from one of the supported medium and then boot the kernel from a USB thumb.
Mainline U-boot does support loading a kernel from a USB thumb.
Regards,
Fabio Estevam