If OSLO_CONFIG_SHOW_CODE_LOCATIONS is set to any non-empty string it
will turn on the ability to see which file has the definition of an
Opt or where set_default() was invoked.
Change-Id: Ie705014dcf331e3c6b3367d2fefbfb9acc091799
Signed-off-by: Doug Hellmann <doug@doughellmann.com>