kbuild: move headers_check rule to usr/include/Makefile

Currently, some sanity checks for uapi headers are done by
scripts/headers_check.pl, which is wired up to the 'headers_check'
target in the top Makefile.

It is true compiling headers has better test coverage, but there
are still several headers excluded from the compile test. I like
to keep headers_check.pl for a while, but we can delete a lot of
code by moving the build rule to usr/include/Makefile.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/Makefile b/Makefile
index a45291d..ad6340a 100644
--- a/Makefile
+++ b/Makefile
@@ -1193,19 +1193,15 @@
 	$(Q)$(MAKE) $(hdr-inst)=include/uapi
 	$(Q)$(MAKE) $(hdr-inst)=arch/$(SRCARCH)/include/uapi
 
+# Deprecated. It is no-op now.
 PHONY += headers_check
-headers_check: headers
-	$(Q)$(MAKE) $(hdr-inst)=include/uapi HDRCHECK=1
-	$(Q)$(MAKE) $(hdr-inst)=arch/$(SRCARCH)/include/uapi HDRCHECK=1
+headers_check:
+	@:
 
 ifdef CONFIG_HEADERS_INSTALL
 prepare: headers
 endif
 
-ifdef CONFIG_HEADERS_CHECK
-all: headers_check
-endif
-
 PHONY += scripts_unifdef
 scripts_unifdef: scripts_basic
 	$(Q)$(MAKE) $(build)=scripts scripts/unifdef
@@ -1473,7 +1469,6 @@
 	@echo  '  versioncheck    - Sanity check on version.h usage'
 	@echo  '  includecheck    - Check for duplicate included header files'
 	@echo  '  export_report   - List the usages of all exported symbols'
-	@echo  '  headers_check   - Sanity check on exported headers'
 	@echo  '  headerdep       - Detect inclusion cycles in headers'
 	@echo  '  coccicheck      - Check with Coccinelle'
 	@echo  ''