cml1.bbclass: Return sorted list of cfg files

Since these files are passed to merge_config.sh its better to use a
sorted list, so we can be sure that this list is always fed in same
order irrespective of python versions on host

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>