scripts/tags.sh: allow to use alternative ctags implementation

Some ctags implementations are available. With this change, You can
specify your favorite one with CTAGS environment variable.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Author: masatake

scripts/tags.sh

@@ -344,7 +344,7 @@ case "$1" in
 
 	"tags")
 		rm -f tags
-		xtags ctags
+		xtags ${CTAGS:-ctags}
 		remove_structs=y
 		;;