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

Reviewed by: imp, kevans
Pull Request: https://github.com/freebsd/freebsd-src/pull/759

The commit, revert, recommit has been squashed down to record the merge
(cherry picked from commit c475e61f66fe8fe939e18ec7821c2340569f3271)
(cherry picked from commit 5fdf01dbeef1f64f8c446561498d662702451ac1)
(cherry picked from commit d3d0b735571d9562812ce5b343a6e91f7a795dbe)