library/certs.c provides some hardcoded certificates that
are used e.g. by the test applications ssl_server2, ssl_client2
in case no certificates are provided on the command line.

The certificates used are from the tests/data_files folder
and have been updated in the latest commits. This commit
updates their copies in certs.c. It also adds comments
indicating the files from which the data is taken, in
order to ease update in the future.