Update makefile in preparation for Documentation/config/*.txt

Update makefile in preparation for Documentation/config/*.txt

config.txt is going to be broken down in smaller pieces and put under
Documentation/config directory. Update build rules to take these files
into account.

A dummy file is added to make sure wildcard expansion is predictable
(depending on shell setting it could expand to nothing or becomes a
path if config directory is empty). The file will be deleted once the
move is over.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>