imx: ventana: make hwconfig initialize based on board configuration

imx: ventana: make hwconfig initialize based on board configuration

The hwconfig env var allows user to control hardware specific configuration
of board specific features but not all Ventana boards have the same features.

We will use the magic default value of "_UNKNOWN_" to signify that the
bootloader should create this based on detected board model.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>