I checked myself and in deed there is no ftdio_sio.ko file in /lib/modules/3.8.7-yocto-standard/kernel/drivers/usb/serial, and I don't know of any image that includes this file, but what's your issue? Do you not see ttyUSB0? If so, I suggest you to add the new device using udev, you can learn how to use udev here: https://wiki.archlinux.org/index.php/udev#Execute_on_USB_insert
However if you're looking for ftdi-sio, AlexT in his repo includes it, you can check it in here: Index of /galileo
Were you able to find and install the ftdio-sio module?