Basic implementation of 'make headers_install'

This adds a make target which exports a subset of headers which contain
definitions which are useful for system libraries and tools. It uses the
BSD 'unifdef' tool to remove instances of #ifdef __KERNEL__, and uses
sed to remove markers like __user.

Based on an original implementation by Arnd Bergmann <arnd@arndb.de>
Hacked about by David Woodhouse <dwmw2@infradead.org>
Reviewed and cleaned up by Sam Ravnborg <sam@ravnborg.org>

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

Makefile

@@ -855,6 +855,17 @@ PHONY += depend dep
 depend dep:
 	@echo '*** Warning: make $@ is unnecessary now.'
 
+# ---------------------------------------------------------------------------
+# Kernel headers
+INSTALL_HDR_PATH=$(MODLIB)/abi
+export INSTALL_HDR_PATH
+
+PHONY += headers_install
+headers_install: include/linux/version.h
+	$(Q)unifdef -Ux /dev/null
+	$(Q)rm -rf $(INSTALL_HDR_PATH)/include
+	$(Q)$(MAKE) -rR -f $(srctree)/scripts/Makefile.headersinst obj=include
+
 # ---------------------------------------------------------------------------
 # Modules
 
@@ -1027,6 +1038,8 @@ help:
 	@echo  '  cscope	  - Generate cscope index'
 	@echo  '  kernelrelease	  - Output the release version string'
 	@echo  '  kernelversion	  - Output the version stored in Makefile'
+	@echo  '  headers_install - Install sanitised kernel headers to INSTALL_HDR_PATH'
+	@echo  '                    (default: /lib/modules/$$VERSION/abi)'
 	@echo  ''
 	@echo  'Static analysers'
 	@echo  '  checkstack      - Generate a list of stack hogs'