Für das X250 gibt's ein Bios-Update. Changelog:

CHANGES IN THIS RELEASE
  Version 1.25

[Important updates]
  Nothing.

[New functions or enhancements]
- Added latest Microsoft revocation file list in the default dbx
  for Secure Boot.

[Problem fixes]
- Provided a workaround to make the PxE boot display resolution to XGA (1024x768)
- Fixed to be able to unlock the password at reboot.
- Fixed an issue where cursor without password prompt might appear at 
  unattended boot.

Aktualisieren geht mit USB Stick wie sonst auch. Läuft gut.