kbuild: collect shorthands into scripts/Kbuild.include

The shorthand "clean" is defined in both the top Makefile and
scripts/Makefile.clean.  Likewise, the "hdr-inst" is defined in
both the top Makefile and scripts/Makefile.headersinst.

To reduce code duplication, this commit collects them into
scripts/Kbuild.include like the "build" and "modbuiltin" shorthands.
It requires scripts/Makefile.clean to include scripts/Kbuild.include,
but its impact on the performance of "make clean" should be
negligible.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>
diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 65e7b08..0f90981 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -179,6 +179,18 @@
 # $(Q)$(MAKE) $(modbuiltin)=dir
 modbuiltin := -f $(srctree)/scripts/Makefile.modbuiltin obj
 
+###
+# Shorthand for $(Q)$(MAKE) -f scripts/Makefile.clean obj=
+# Usage:
+# $(Q)$(MAKE) $(clean)=dir
+clean := -f $(srctree)/scripts/Makefile.clean obj
+
+###
+# Shorthand for $(Q)$(MAKE) -rR -f scripts/Makefile.headersinst obj=
+# Usage:
+# $(Q)$(MAKE) $(hdr-inst)=dir
+hdr-inst := -rR -f $(srctree)/scripts/Makefile.headersinst obj
+
 # Prefix -I with $(srctree) if it is not an absolute path.
 # skip if -I has no parameter
 addtree = $(if $(patsubst -I%,%,$(1)), \