This adds the target guide for Netduino 2, Netduino Plus 2 and STM32VLDISCOVERY.

Signed-off-by: Alexandre Iooss <erdnaxe@crans.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210617165647.2575955-4-erdnaxe@crans.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>