u-blox TOBY-L2 support

The u-blox TOBY-L2 is a series of LTE Cat 4 modules with HSPA+ and/or 2G fallback. Here's an example configuration:

VintageNet.configure(
  "ppp0",
  %{
    type: VintageNetMobile,
    vintage_net_mobile: %{
      modem: VintageNetMobile.Modem.UbloxTOBYL2,
      service_providers: [
        %{apn: "lte-apn", usage: :eps_bearer},
        %{apn: "old-apn", usage: :pdp}
      ]
    }
  }
)

This implementation currently requires APNs to be annotated for whether they are to be used on LTE (:eps_bearer) or on UMTS/GPRS (:pdp).

Required Linux kernel options

• CONFIG_USB_SERIAL=m
• CONFIG_USB_SERIAL_WWAN=m
• CONFIG_USB_SERIAL_OPTION=m

Required modem preparation

The Toby L2 is a composite USB device that can be configured to expose various different interfaces. By default, it has one CDC ACM interface. This implementation requires two, so you have to send it the following over a tty interface (via Circuits.UART or externally):

AT+UUSBCONF=2

That command is saved NVRAM and only needs to be sent once. See section "19.17 USB profiles configuration +UUSBCONF" in the u-blox AT commands manual