[Electron] retrieve IMEI and ICCID

Firmware
2

This did it for me (Electron with 0.5.2 FW):

  CellularDevice device;
  memset(&device, 0, sizeof(device));
  device.size = sizeof(device);
  cellular_device_info(&device, NULL);

  //device.imei (string) IMEI of the 3G modem
  //device.iccid (string) ICCID of the SIM card