kbuild: fix make TAGS (for emacs use)

From: bongiojp@clarkson.edu <Jeremy Bongio>
make TAGS does not make source code tags for emacs. It instead
returns an error than "etags -" isn't valid. The problem is
easily remedied.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -1203,9 +1203,15 @@ cscope: FORCE
 	$(call cmd,cscope)
 
 quiet_cmd_TAGS = MAKE   $@
-cmd_TAGS = $(all-sources) | etags -
+define cmd_TAGS
+	rm -f $@; \
+	ETAGSF=`etags --version | grep -i exuberant >/dev/null && echo "-I __initdata,__exitdata,EXPORT_SYMBOL,EXPORT_SYMBOL_GPL --extra=+f"`; \
+	$(all-sources) | xargs etags $$ETAGSF -a
+endef
+
+TAGS: FORCE
+	$(call cmd,TAGS)
 
-# 	Exuberant ctags works better with -I
 
 quiet_cmd_tags = MAKE   $@
 define cmd_tags
@@ -1214,9 +1220,6 @@ define cmd_tags
 	$(all-sources) | xargs ctags $$CTAGSF -a
 endef
 
-TAGS: FORCE
-	$(call cmd,TAGS)
-
 tags: FORCE
 	$(call cmd,tags)