kbuild: fix false positive warning/error about missing libelf

[ Upstream commit ef7cfd00b2caf6edeb7f169682b64be2d0a798cf ]

For the same reason as commit 25896d073d8a ("x86/build: Fix compiler
support check for CONFIG_RETPOLINE"), you cannot put this $(error ...)
into the parse stage of the top Makefile.

Perhaps I'd propose a more sophisticated solution later, but this is
the best I can do for now.

Link: https://lkml.org/lkml/2017/12/25/211
Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Reported-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reported-by: Qian Cai <cai@lca.pw>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Qian Cai <cai@lca.pw>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Makefile

@@ -954,11 +954,6 @@ ifdef CONFIG_STACK_VALIDATION
   ifeq ($(has_libelf),1)
     objtool_target := tools/objtool FORCE
   else
-    ifdef CONFIG_UNWINDER_ORC
-      $(error "Cannot generate ORC metadata for CONFIG_UNWINDER_ORC=y, please install libelf-dev, libelf-devel or elfutils-libelf-devel")
-    else
-      $(warning "Cannot use CONFIG_STACK_VALIDATION=y, please install libelf-dev, libelf-devel or elfutils-libelf-devel")
-    endif
     SKIP_STACK_VALIDATION := 1
     export SKIP_STACK_VALIDATION
   endif
@@ -1115,6 +1110,14 @@ uapi-asm-generic:
 
 PHONY += prepare-objtool
 prepare-objtool: $(objtool_target)
+ifeq ($(SKIP_STACK_VALIDATION),1)
+ifdef CONFIG_UNWINDER_ORC
+	@echo "error: Cannot generate ORC metadata for CONFIG_UNWINDER_ORC=y, please install libelf-dev, libelf-devel or elfutils-libelf-devel" >&2
+	@false
+else
+	@echo "warning: Cannot use CONFIG_STACK_VALIDATION=y, please install libelf-dev, libelf-devel or elfutils-libelf-devel" >&2
+endif
+endif
 
 # Generate some files
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