Set secure_boot flag in Kernel Zero-Page (#7482)

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)