package/wireguard-linux-compat: new package

With the kernel support for WireGuard getting mainlined, the upstream repo
has been split in a wireguard-tools repo for the userspace tooling and
wireguard-linux-compat for the kernel side (for 3.10+ legacy kernels).

Add a wireguard-linux-compat for the compatibility out-of-tree kernel
module, and update the Config.in.legacy handling to use it.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>