Actually all of the kernel code is licensed with GPL2, but I don't know of any parts where the FSF is the copyright holder.
For instance is FSF copyright holder of BusyBox. I didn't check further as that is sufficient to demand the sourcecode of the kernel. (GPL2 demands to release all the sourcecode which is used in a product where a GPL2 part is used.)
The FSF ask all authors to place a copyright notice in the source so if FSF is copyright holder you can always see it in the source.
Anyway I contacted AVM about the source of the FB7390 version 6.01 international firmware. They answered that the source is not online yet and asked for a bit patience. Although this is a violation of the license, this sound reasonable to me. I contacted them again for an indication when the source will be available. So hopefully I get an answer on that question too.