kbuild: Add support for an "archheaders" target

Add support for an "archheaders" target.  This target can generate
files that need to be installed for user space by "make
headers_install" or "make headers_install_all".

In order to support "make headers_install_all", it must be able to run
without the tree having to be configured first.

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

Makefile

@@ -442,7 +442,7 @@ asm-generic:
 
 no-dot-config-targets := clean mrproper distclean \
 			 cscope gtags TAGS tags help %docs check% coccicheck \
-			 include/linux/version.h headers_% \
+			 include/linux/version.h headers_% archheaders \
 			 kernelversion %src-pkg
 
 config-targets := 0
@@ -979,7 +979,7 @@ prepare1: prepare2 include/linux/version.h include/generated/utsrelease.h \
                    include/config/auto.conf
 	$(cmd_crmodverdir)
 
-archprepare: prepare1 scripts_basic
+archprepare: archheaders prepare1 scripts_basic
 
 prepare0: archprepare FORCE
 	$(Q)$(MAKE) $(build)=.
@@ -1046,8 +1046,11 @@ hdr-inst := -rR -f $(srctree)/scripts/Makefile.headersinst obj
 # If we do an all arch process set dst to asm-$(hdr-arch)
 hdr-dst = $(if $(KBUILD_HEADERS), dst=include/asm-$(hdr-arch), dst=include/asm)
 
+PHONY += archheaders
+archheaders:
+
 PHONY += __headers
-__headers: include/linux/version.h scripts_basic asm-generic FORCE
+__headers: include/linux/version.h scripts_basic asm-generic archheaders FORCE
 	$(Q)$(MAKE) $(build)=scripts build_unifdef
 
 PHONY += headers_install_all