UEFI machines can be booted in Secure Boot mode.  Add a EFI_SECURE_BOOT bit
that can be passed to efi_enabled() to find out whether secure boot is
enabled.

This will be used by the SysRq+x handler, registered by the x86 arch, to find
out whether secure boot mode is enabled so that it can be disabled.
Signed-off-by: Josh Boyer <jwboyer@fedoraproject.org>
Signed-off-by: David Howells <dhowells@redhat.com>