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>