samples: net: echo_server: Add VLAN support

If user enables VLAN support, then the sample application will
create two extra VLAN network interfaces for testing purposes.

The application can be compiled like this for VLAN support:

  cmake -DBOARD=qemu_x86 -DOVERLAY_CONFIG=overlay-vlan.conf ..

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>