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.

So the sequencing is now as follow:

1. Bootstrap: /boot/defaults/loader.conf
2. Read loader_conf_files files: /boot/device.hints /boot/loader.conf
3. Read loader_conf_dirs files: /boot/loader.conf.d/*.conf
4. And finally, rread local_loader_conf_files files: /boot/loader.conf.local