A big potential customer is reportedly ready to firm up its order.Air taxi start-up Lilium (LILM 5.47%) has reportedly secured a firm order from a state-owned Saudi Arabian airline. Investors appreciate the vote of confidence, sending Lilium shares up about 10% as of 11:30 a.m. ET.Support, and potentially cash?Lilium is one of several start-ups working to develop and commercialize so-called "eVTOL" airframes, shorthand for electric vertical takeoff and landing planes. The aircraft are designed to take off a ...