imx7d-sdb has been converted to use disto boot in U-Boot.
Add extlinux.conf support so that the board can boot correctly.
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Pierre-Jean Texier <texier.pj2@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
