colibri_vf: use UUID for rootfs

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.

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