Remove the "version" parameter in order to make it more widely
available.
Starts making some kernel configuration helpers available.
The intent is to be able to better build and check the linux kernel
configuration.
Also move to a pkgs/os-specific/linux/kernel/linux-mptcp-94.nix. makes
updates easier.
Added a structured config pkgs/os-specific/linux/kernel/mptcp-config.nix
that can be shared between the different kernels.