pico-imx6ul: Define partition layout in the environment

Create layout with a boot 16MiB partition and rootfs with remain
space.

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>