Setting the secure_boot flag, avoids getting the printout "EFI stub: UEFI Secure Boot is enabled." when booting a Linux kernel with linuxx64.efi.stub and EFI SecureBoot enabled. This is mainly a cosmetic fixup, as the "quiet" kernel parameter does not silence pr_efi printouts in the linux kernel (this only works using the efi stub from the linux source tree)