Add "ait install-unlocked" variant of LineageOS install

There should be a variant of the "ait install" LineageOS installation for the "ait install-unlocked" section, i.e. which doesn't require a running OS, but assumes an unlocked device and starts the installation from bootloader mode.

The implementation is as easy as skipping the first part of the the classic installation where the model name is determined, the device is unlocked and the device is rebooted into bootloader mode.

Among others, this is useful to install LineageOS on a device where a previous (manual) installation attempt failed, i.e. to "unbrick" a device which is unusable but not really bricked.