colibri_imx6: use UUID for rootfs

1. Replace usage of "/dev/mmcblk*p*" with a proper UUID of rootfs
partition. This fixes the issue, when MMC controllers are probed in
a different order in U-boot and Linux kernel.
2. Fix legacy USB command (both sdboot and usbboot can be used now).

Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>