update config for USB gadget (systemd) support on Rpi

[Antoniu Miclăuş]

With:
analogdevicesinc/linux_image_ADI-scripts#27
USB gadget support was added with systemd.

For this to work properly on Rpi the following lines need to be added in:

1. config.txt -> dtoverlay=dwc2
2. cmdline.txt -> modules-load=dwc2