nanobsd: Add a provisional populate_part function

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.

Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48790