mx6cuboxi: Use PARTUUID to specify the rootfs location

mx6cuboxi: Use PARTUUID to specify the rootfs location

Currently the rootfs location is passed via mmcblk number and the
problem with this approach is that the mmcblk number for the eMMC
changes depending on the kernel version.

In order to avoid such issue, use UUID method to specify the rootfs
location.

This change was made based on U-Boot commit:

  - ca4f338e2efece5196eb2178e5f7d07be828da6e

Signed-off-by: Fabio Berton <fabio.berton@ossystems.com.br>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>