wireguard: add wireguard to base packages

Move wireguard from openwrt/packages to base a package.

This follows the pattern of kmod-cake and openvpn. Cake is a fast-moving
experimental kernel module that many find essential and useful. The
other is a VPN client. Both are inside of core. When you combine the two
characteristics, you get WireGuard. Generally speaking, because of the
extremely lightweight nature and "stateless" configuration of WireGuard,
many view it as a core and essential utility, initiated at boot time
and immediately configured by netifd, much like the use of things like
GRE tunnels.

WireGuard has a backwards and forwards compatible Netlink API, which
means the userspace tools should work with both newer and older kernels
as things change. There should be no versioning requirements, therefore,
between kernel bumps and userspace package bumps.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Jo-Philipp Wich <jo@mein.io>
Acked-by: Felix Fietkau <nbd@nbd.name>