[NAME], I can now confirm for you that Dell is working on a new BIOS to resolve the boot issue caused by 1.2.3. I'm trying to get an ETA for the new BIOS version. I'll get back as soon as I know more. 
Dell XPS 13 9360, previously running dual boot Windows 10 and Ubuntu 16.04.1 LTS. After upgrading to 1.2.3 BIOS, the laptop fails to boot. I receive a BIOS message 'No boot device found'. This is using AHCI for the drive configuration. If I change the drive configuration to RAID instead (and nothing else), I receive the error 'Inaccessible boot device' from Windows 10 -- which isn't hugely surprising as it's configured for AHCI and doesn't have the intel RST driver installed. I've tried downgrading the BIOS to 1.0.7 but received a message 'Incompatible downgrade'. Any ideas as to why it can "see" the boot device in RAID mode, but not in AHCI?
XPS 13 9360 -- 1.2.3 BIOS results in 'no boot device found'