Don't advertise (or allow) headers_{install,check} where inappropriate.

For architectures which don't have the include/asm-$(ARCH)/Kbuild file,
like ARM26, UM, etc.
Signed-off-by: David Woodhouse <dwmw2@infradead.org>

Makefile

@@ -894,6 +894,9 @@ export INSTALL_HDR_PATH
 
 PHONY += headers_install
 headers_install: include/linux/version.h
+	@if [ ! -r include/asm-$(ARCH)/Kbuild ]; then \
+	  echo '*** Error: Headers not exportable for this architecture ($(ARCH))'; \
+	  exit 1 ; fi
 	$(Q)unifdef -Ux /dev/null
 	$(Q)rm -rf $(INSTALL_HDR_PATH)/include
 	$(Q)$(MAKE) -rR -f $(srctree)/scripts/Makefile.headersinst obj=include
@@ -1076,13 +1079,17 @@ help:
 	@echo  '  cscope	  - Generate cscope index'
 	@echo  '  kernelrelease	  - Output the release version string'
 	@echo  '  kernelversion	  - Output the version stored in Makefile'
-	@echo  '  headers_install - Install sanitised kernel headers to INSTALL_HDR_PATH'
+	@if [ -r include/asm-$(ARCH)/Kbuild ]; then \
+	 echo  '  headers_install - Install sanitised kernel headers to INSTALL_HDR_PATH'; \
+	 fi
 	@echo  '                    (default: $(INSTALL_HDR_PATH))'
 	@echo  ''
 	@echo  'Static analysers'
 	@echo  '  checkstack      - Generate a list of stack hogs'
 	@echo  '  namespacecheck  - Name space analysis on compiled kernel'
-	@echo  '  headers_check   - Sanity check on exported headers'
+	@if [ -r include/asm-$(ARCH)/Kbuild ]; then \
+	 echo  '  headers_check   - Sanity check on exported headers'; \
+	 fi
 	@echo  ''
 	@echo  'Kernel packaging:'
 	@$(MAKE) $(build)=$(package-dir) help