kbuild: build all prerequisites of headers_install simultaneously

Currently, scripts/unifdef is compiled after scripts_basic,
uapi-asm-generic, archheaders, and archscripts.

The proper dependency is just scripts_basic. There is no problem
to compile scripts/unifdef and other headers at the same time.

Split scripts_unifdef out in order to allow more parallel building.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1181,8 +1181,7 @@ export INSTALL_HDR_PATH = $(objtree)/usr
 PHONY += archheaders archscripts
 
 PHONY += __headers
-__headers: $(version_h) scripts_basic uapi-asm-generic archheaders archscripts
-	$(Q)$(MAKE) $(build)=scripts scripts/unifdef
+__headers: $(version_h) scripts_unifdef uapi-asm-generic archheaders archscripts
 
 PHONY += headers_install
 headers_install: __headers
@@ -1204,6 +1203,10 @@ ifdef CONFIG_HEADERS_CHECK
 all: headers_check
 endif
 
+PHONY += scripts_unifdef
+scripts_unifdef: scripts_basic
+	$(Q)$(MAKE) $(build)=scripts scripts/unifdef
+
 # ---------------------------------------------------------------------------
 # Kernel selftest