This commit enables On-the-go Ethernet over the USB-C connector. This
allows the user to simply connect their laptop to the Pi using a USB-C
cable, get an IP address on the USB interface and immediately start
working on the Pi, reducing setup time.

The following changes were made:

- Enable dwc2, which is a USB driver handling the OTG events. This also
  required changing the way the config.txt file is generated.
- Enable g_ether, which is the USB gadget driver for USB Ethernet.
- Configure systemd to start a DHCP server on usb0.