qmi-firmware-update: when udev not used, loop waiting for the device reboot

The process to upgrade the modems using qmi-firmware-update in openwrt is tedious and error-prone, as the user needs to manually select firmware preference settings, reboot the device, wait for the device to reboot in download mode, make sure it's really in download mode, and then select the correct TTY in the system.

All these steps are done automatically by qmi-firmware-update when udev is available, and we could also try to do them without udev, by performing manual checks e.g. every 5s, instead of relying on the kernel-reported events. This method would be much more generic than e.g. relying on openwrt hotplug events, which would be much more openwrt specific.