In order to allow D-Bus usage during early boot (where /usr is not
accessible) also search for bus activation files in
/lib/dbus-1/system-services/. This is only a first step in the right
direction, before we really can boot without /usr we'd need to move all
current activation files (or possibly replace
/usr/dbus-1/system-services to a symlink to
/lib/dbus-1/system-services).