kbuild: add kconfig symbols to tags output

Steps to reproduce:

	vi -t NETFILTER
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -1396,7 +1396,7 @@ define xtags
 	    $(all-kconfigs) | xargs $1 -a \
 		--langdef=kconfig \
 		--language-force=kconfig \
-		--regex-kconfig='/^[[:blank:]]*config[[:blank:]]+([[:alnum:]_]+)/\1/'; \
+		--regex-kconfig='/^[[:blank:]]*(menu|)config[[:blank:]]+([[:alnum:]_]+)/\2/'; \
 	    $(all-defconfigs) | xargs -r $1 -a \
 		--langdef=dotconfig \
 		--language-force=dotconfig \
@@ -1404,7 +1404,7 @@ define xtags
 	elif $1 --version 2>&1 | grep -iq emacs; then \
 	    $(all-sources) | xargs $1 -a; \
 	    $(all-kconfigs) | xargs $1 -a \
-		--regex='/^[ \t]*config[ \t]+\([a-zA-Z0-9_]+\)/\1/'; \
+		--regex='/^[ \t]*(menu|)config[ \t]+\([a-zA-Z0-9_]+\)/\2/'; \
 	    $(all-defconfigs) | xargs -r $1 -a \
 		--regex='/^#?[ \t]?\(CONFIG_[a-zA-Z0-9_]+\)/\1/'; \
 	else \