Comment 43 for bug 1789650

AFAIK, BootCurrent is the only way to know how the system booted. OTOH, I suppose that, in theory, you could have MAAS's GRUB write a file somewhere that indicates a MAAS/PXE boot. You could then look for that file (checking its time stamp to be sure it was recent) -- but if BootCurrent is missing for GRUB in the EFI environment as well as for the kernel, that wouldn't help you figure out which of the Boot#### entries was used.