nitrogen6x: update video console name

After migration to DM 'vga' name is not longer supported,
Update the upgrade script to use 'vidconsole' instead.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Troy Kisky <troy.kisky@boundarydevices.com>
Acked-by: Troy Kisky <troy.kisky@boundarydevices.com>

board/boundary/nitrogen6x/6x_upgrade.txt

@@ -1,4 +1,4 @@
-setenv stdout serial,vga
+setenv stdout serial,vidconsole
 echo "check U-Boot" ;
 setenv offset 0x400
 if ${fs}load ${dtype} ${disk}:1 12000000 u-boot.imx || ${fs}load ${dtype} ${disk}:1 12000000 u-boot.nopadding ; then