[bootloader] Khi slot alert

[Alerymin]

Is your feature request related to a problem? Please describe.
KhiCAS, when launched, wants a certified boot, and we can update it when it starts. It works only on Khi. However, if we run Khi and update the certified boot, Upsilon bootloader gets erased

Describe the solution you'd like
A warning, similar to Epsilon's one, to tell people not to update the boot when starting KhiCAS