From the header of '/etc/mysql/my.cnf':
MariaDB programs look for option files in a set of
locations which depend on the deployment platform.
[...] For information about these locations, do:
'my_print_defaults --help' and see what is printed under
"Default options are read from the following files in the given order:"
More information at: http://dev.mysql.com/doc/mysql/en/option-files.html