efi/libstub/arm: Add opt-in Kconfig option for the DTB loader

There are various ways a platform can provide a device tree binary
to the kernel, with different levels of sophistication:

- ideally, the UEFI firmware, which is tightly coupled with the
  platform, provides a device tree image directly as a UEFI
  configuration table, and typically permits the contents to be
  manipulated either via menu options or via UEFI environment
  variables that specify a replacement image,

- GRUB for ARM has a 'devicetree' directive which allows a device
  tree image to be loaded from any location accessible to GRUB, and
  supersede the one provided by the firmware,

- the EFI stub implements a dtb= command line option that allows a
  device tree image to be loaded from a file residing in the same
  file system as the one the kernel image was loaded from.

The dtb= command line option was never intended to be more than a
development feature, to allow the other options to be implemented
in parallel. So let's make it an opt-in feature that is disabled
by default, but can be re-enabled at will.

Note that we already disable the dtb= command line option when we
detect that we are running with UEFI Secure Boot enabled.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Alexander Graf <agraf@suse.de>
Acked-by: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20180711094040.12506-7-ard.biesheuvel@linaro.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>