efi: Lock down the kernel if booted in secure boot mode

efi: Lock down the kernel if booted in secure boot mode

Based on an earlier patch by David Howells, who wrote the following
description:

> UEFI Secure Boot provides a mechanism for ensuring that the firmware will
> only load signed bootloaders and kernels.  Certain use cases may also
> require that all kernel modules also be signed.  Add a configuration option
> that to lock down the kernel - which includes requiring validly signed
> modules - if the kernel is secure-booted.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
[Salvatore Bonaccorso: After fixing https://bugs.debian.org/956197 the
help text for LOCK_DOWN_IN_EFI_SECURE_BOOT was adjusted to mention that
lockdown is triggered in integrity mode (https://bugs.debian.org/1025417)]
Signed-off-by: Salvatore Bonaccorso <carnil@debian.org>
Gbp-Pq: Topic features/all/lockdown
Gbp-Pq: Name efi-lock-down-the-kernel-if-booted-in-secure-boot-mo.patch