pico-imx6ul: Use PARTUUID to specify the rootfs location

Currently the rootfs location is passed via mmcblk number and using
the UUID method to specify the rootfs location is a better approach
working even if mmcblk number for the eMMC changes depending on the
kernel versions.

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>