package/wireguard: change to the wireguard-tools 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).

So rename the package to wireguard-tools, use the new upstream and drop the
kernel module handling.

Also add Config.in.legacy handling for existing users.

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