dragonboard410c: prefer sdcard boot over emmc

Make the external devices the preferred ones when booting the system
(usb is already the first option). This allows users to easily boot
custom distributions without requiring them to reflash/customize u-boot.

Cc: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Signed-off-by: Ricardo Salveti <rsalveti@rsalveti.net>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Tested-by: Andreas Färber <afaerber@suse.de>
Acked-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>