efi/libstub: Treat missing SecureBoot variable as Secure Boot disabled

The newly refactored code that infers the firmware's Secure Boot state
prints the following error when the EFI variable 'SecureBoot' does not
exist:

  EFI stub: ERROR: Could not determine UEFI Secure Boot status.

However, this variable is only guaranteed to be defined on a system that
is Secure Boot capable to begin with, and so it is not an error if it is
missing. So report Secure Boot as being disabled in this case, without
printing any error messages.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/1488395076-29712-2-git-send-email-ard.biesheuvel@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>