efi: avoid fortify checks in EFI stub

This avoids CONFIG_FORTIFY_SOURCE from being enabled during the EFI stub
build, as adding a panic() implementation may not work well. This can
be adjusted in the future.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Kees Cook <[e-mail]>
Suggested-by: Daniel Micay <[e-mail]>
Reviewed-by: Ard Biesheuvel <[e-mail]>
Acked-by: Mark Rutland <[e-mail]>
Cc: Matt Fleming <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/firmware/efi/libstub/Makefile

···
 cflags-$(CONFIG_EFI_ARMSTUB)	+= -I$(srctree)/scripts/dtc/libfdt
 
 KBUILD_CFLAGS			:= $(cflags-y) -DDISABLE_BRANCH_PROFILING \
+				   -D__NO_FORTIFY \
 				   $(call cc-option,-ffreestanding) \
 				   $(call cc-option,-fno-stack-protector)