kbuild: move hdr-inst shorthand to top Makefile

Now that hdr-inst is used only in the top Makefile, move it there
from scripts/Kbuild.include.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1190,6 +1190,8 @@ headers_install: headers
 
 PHONY += archheaders archscripts
 
+hdr-inst := -f $(srctree)/scripts/Makefile.headersinst obj
+
 PHONY += headers
 headers: $(version_h) scripts_unifdef uapi-asm-generic archheaders archscripts
 	$(if $(wildcard $(srctree)/arch/$(SRCARCH)/include/uapi/asm/Kbuild),, \

scripts/Kbuild.include

@@ -187,12 +187,6 @@ dtbinst := -f $(srctree)/scripts/Makefile.dtbinst obj
 # $(Q)$(MAKE) $(clean)=dir
 clean := -f $(srctree)/scripts/Makefile.clean obj
 
-###
-# Shorthand for $(Q)$(MAKE) -f scripts/Makefile.headersinst obj=
-# Usage:
-# $(Q)$(MAKE) $(hdr-inst)=dir
-hdr-inst := -f $(srctree)/scripts/Makefile.headersinst obj
-
 # echo command.
 # Short version is used, if $(quiet) equals `quiet_', otherwise full one.
 echo-cmd = $(if $($(quiet)cmd_$(1)),\