configs: imx8mn_bsh_smm_s2: add UBI commands

imx8mn_bsh_smm_s2 uses ubifs rootfs, UBI commands are required to flash
it.

Co-developed-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>