samples: net: echo-client: Add VLAN support

Add support for VLANs which are disabled by default.
The application can be configured to use the VLANs by setting
the IP addresses properly in config file. The VLAN support in
this sample application is only meant for testing multiple network
interface handling.

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>