mx6slevk: Use PARTUUID to specify the rootfs location

mx6slevk can run different kernel versions, such as NXP 4.1 or mainline.

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

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

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>