board: toradex: make USB PID from config block optional

board: toradex: make USB PID from config block optional

If config block support is enabled, USB gadget modes unconditionally
use Toradex Product ID as USB PID. Some applications might prefer a
different and/or static USB PID. Add a Kconfig configuration option
to descide whether to use USB PID from config block or the fallback
config option CONFIG_G_DNL_PRODUCT_NUM.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>