nanobsd: Add a provisional populate_part function

[jlduran]

Summary

Add a _populate_part(ition) function that mimics the current
populate_slice. Note however, that this function is not
backward-compatible with populate_slice, hence the different name. A
"_" is prepended to signal that it still experimental.

It can be used to populate the /cfg and /data partition using makefs(8).

Initially not wired.

TODO:

• Related: https://github.com/freebsd/poudriere/issues/1181 With this approach, an mtree file will be required if a mix of different users/groups is desired.

[jlduran]

• Actually remove the directory (add -r)
• Do not overwrite previous traps (otherwise the temporary /cfg directory is not removed).