Comment 620 for bug 1734147

Marek, Secure Boot is a UEFI feature. It is not possible for you to have Secure Boot blocking a kernel boot if you do not have UEFI.

It is possible that your EFI System Partition is not mounted at /boot/efi, if you have modified your /etc/fstab since installation. For the system to boot at all under UEFI, you must have an EFI System Partition. For you to have a grub that refuses to boot unsigned kernels, you must have updated the contents on that EFI System Partition rather recently.

The output of 'efibootmgr -v' may help you locate the partition so that you can mount it.