usb: gadget: fastboot: Set the Serial Number for Fastboot Gadget

Configure the serial number using the serial# environment variable
during the fastboot bind.

This enables "fastboot devices" to return the serial number for
the attached devices.

Signed-off-by: Dileep Katta <dileep.katta@linaro.org>
Acked-by: Steve Rae <srae@broadcom.com>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>