This introduces the device parameters:

usbgadget.product
usbgadget.vendor
usbgadget.manufacturer
usbgadget.productname

These variables are used to configure the USB vendor id, product id,
manufacturer name and product name.

Previously these were configured with arguments to the usbserial
and dfu command. The parameters are device static, so it's nice to
configure it somewhere in the environment instead of when calling
dfu/usbserial. Also when other gadget drivers are added we do not
have to duplicate the option parsing further.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>