kbuild: add CONFIG_ASM_MODVERSIONS

Add CONFIG_ASM_MODVERSIONS. This allows to remove one if-conditional
nesting in scripts/Makefile.build.

scripts/Makefile.build is run every time Kbuild descends into a
sub-directory. So, I want to avoid $(wildcard ...) evaluation
where possible although computing $(wildcard ...) is so cheap that
it may not make measurable performance difference.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 10adf3b..9fa0936 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -349,11 +349,7 @@
 quiet_cmd_as_o_S = AS $(quiet_modtag)  $@
       cmd_as_o_S = $(CC) $(a_flags) -c -o $@ $<
 
-ifdef CONFIG_MODVERSIONS
-
-ASM_PROTOTYPES := $(wildcard $(srctree)/arch/$(SRCARCH)/include/asm/asm-prototypes.h)
-
-ifneq ($(ASM_PROTOTYPES),)
+ifdef CONFIG_ASM_MODVERSIONS
 
 # versioning matches the C process described above, with difference that
 # we parse asm-prototypes.h C header to get function definitions.
@@ -369,7 +365,6 @@
 		rm -f $(@D)/.tmp_$(@F:.o=.ver);					\
 	fi
 endif
-endif
 
 $(obj)/%.o: $(src)/%.S $(objtool_dep) FORCE
 	$(call if_changed_rule,as_o_S)