nanobsd: Add a provisional populate /cfg function

[jlduran]

Summary

Add a provisional _populate_cfg_part function. It populates the /cfg
partition, but using makefs(8), which is more in-line with what release
engineering uses to create images.

Test Plan

1. The default, an empty /cfg slice:
   
   $ sh nanobsd.sh -U

2. A custom /cfg slice (~/nanobsd/test/cfg/) without a metalog file (default user/group):
   
   $ mkdir -p ~/nanobsd/test $ echo "NANO_CFGDIR=$HOME/nanobsd/test/cfg" > ~/nanobsd/test/config.nano $ sh nanobsd.sh -U -c ~/nanobsd/test/config.nano

3. A custom /cfg slice (~/nanobsd/test/cfg/) with a metalog file (~/nanobsd/test/metalog_cfg):
   
   $ mkdir -p ~/nanobsd/test $ echo "NANO_CFGDIR=$HOME/nanobsd/test/cfg" > ~/nanobsd/test/config.nano $ echo "NANO_METALOG_CFG=$HOME/nanobsd/test/metalog_cfg" >> ~/nanobsd/test/config.nano $ sh nanobsd.sh -U -c ~/nanobsd/test/config.nano

NOTE: Initially not wired, the -U flag is available later in the commit series.