efi: Add secure boot detection

Introduce grub_efi_get_secureboot() function which returns whether
UEFI Secure Boot is enabled or not on UEFI systems.

Signed-off-by: Ignat Korchagin <ignat@cloudflare.com>
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: Marco A Benatto <mbenatto@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

include/grub/efi/sb.h

@@ -0,0 +1,40 @@
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2020  Free Software Foundation, Inc.
+ *
+ *  GRUB is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  GRUB is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef GRUB_EFI_SB_H
+#define GRUB_EFI_SB_H     1
+
+#include <grub/types.h>
+#include <grub/dl.h>
+
+#define GRUB_EFI_SECUREBOOT_MODE_UNSET	0
+#define GRUB_EFI_SECUREBOOT_MODE_UNKNOWN	1
+#define GRUB_EFI_SECUREBOOT_MODE_DISABLED	2
+#define GRUB_EFI_SECUREBOOT_MODE_ENABLED	3
+
+#ifdef GRUB_MACHINE_EFI
+extern grub_uint8_t
+EXPORT_FUNC (grub_efi_get_secureboot) (void);
+#else
+static inline grub_uint8_t
+grub_efi_get_secureboot (void)
+{
+  return GRUB_EFI_SECUREBOOT_MODE_UNSET;
+}
+#endif
+#endif /* GRUB_EFI_SB_H */