kernel: add kmod-vrf

Add option to compile kmod-vrf, support for Virtual Routing and
Forwarding (Lite).

This module depends on NET_L3_MASTER_DEV, which is a boolean kernel
option, so we need to create a configuration option also for this, and
make kmod-vrf depend on it.

Signed-off-by: Marek Behún <kabel@kernel.org>