Since I was unable to find any info in the documentation or forums on this topic, I'm sharing my findings here. We did get boot to a M.2 SSD in RAID1 to boot ESXi on a new UCS Mini/FlashStack deployment with the new "UCS-M2-HWRAID Boot-Optimized M.2 RAID Controller". Unlike the previous method that wouldn't allow boot from a SWRAID M.2 SSD, we can now use hardware RAID with ESXi (like we used to in the old days with SATA disks and RAID controller). They are not much more expensive than SD cards so Cisco SEs have been recommending M.2 instead. Anyway...Similar to configuring C or S-Series, you've got to build Storage Policies & Profiles to create a LUN. Then UEFI is required along with the UEFI boot parameters. See screenshots for the config that worked. This article has some useful tips on UEFI boot, but is outdated when it comes to the new HWRAID module and being able to boot via RAID1.