stand/lua: always allow overriding with local config files
Loader now also read configuration files listed in local_loader_conf_files.
Files listed here are the last ones read. And /boot/loader.conf.local was
moved from loader_conf_files to local_loader_conf_files leaving only
loader.conf and device.hints in loader_conf_files by default.
The idea is to ensure local_loader_conf_files, i.e., /boot/loader.conf.local,
can always be used to override other user defined settings.