[libssu] Implement Mer HA-based model identification

Adaptations based on mer-hybris should store the device name of the
Droid device in /etc/os-release under the vendor MER_HA_DEVICE key.
[ssudeviceinfo] Remove obsolete QtSystems dependency

Our QtSystems depends on libssu now for the device model,
so we should not make QtSystems depend on libssu to avoid
infinite recursion.