The company confirmed today it's working on a new NVMe RAID driver that will enable bootable RAID modes 0, 1 and 10 for up to ten devices on X399 motherboards.
This new driver will be offered for free and is expected to be available on September 25, 2017. It will be implemented via an UEFI update so you will need to rely on good support from your motherboard maker to get this functionality on time.