efi: Add EFI shim lock verifier

This module provides shim lock verification for various kernels
if UEFI secure boot is enabled on a machine.

It is recommended to put this module into GRUB2 standalone image
(avoid putting iorw and memrw modules into it; they are disallowed
if UEFI secure boot is enabled). However, it is also possible to use
it as a normal module. Though such configurations are more fragile
and less secure due to various limitations.

If the module is loaded and UEFI secure boot is enabled then:
  - module itself cannot be unloaded (persistent module),
  - the iorw and memrw modules cannot be loaded,
  - if the iorw and memrw modules are loaded then
    machine boot is disabled,
  - GRUB2 defers modules and ACPI tables verification to
    other verifiers.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Ross Philipson <ross.philipson@oracle.com>

grub-core/Makefile.core.def

@@ -899,6 +899,12 @@ module = {
   common = commands/verifiers.c;
 };
 
+module = {
+  name = shim_lock;
+  common = commands/efi/shim_lock.c;
+  enable = x86_64_efi;
+};
+
 module = {
   name = hdparm;
   common = commands/hdparm.c;