Sort the contents of files-in-image.txt to avoid unnecessary changes
showing up in the history due to reordering.
(From OE-Core rev: 791ad7a522f6800114febd1605072202c1450356)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>