nanobsd: Add a provisional populate /data function

[jlduran]

Summary

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

Test Plan

1. The default, no /data slice:
   
   $ sh nanobsd.sh -U

2. A custom /data slice without a metalog file (default user/group):
   
   $ mkdir -p ~/nanobsd/test $ echo "NANO_DATADIR=$HOME/nanobsd/test/data" > ~/nanobsd/test/config.nano $ echo "NANO_DATASIZE=2000000" >> ~/nanobsd/test/config.nano $ sh nanobsd.sh -U -c ~/nanobsd/test/config.nano

3. A custom /data slice with a metalog file:
   
   $ mkdir -p ~/nanobsd/test $ echo "NANO_DATADIR=$HOME/nanobsd/test/data" > ~/nanobsd/test/config.nano $ echo "NANO_DATASIZE=2000000" >> ~/nanobsd/test/config.nano $ echo "NANO_METALOG_DATA=$HOME/nanobsd/test/metalog_data" >> ~/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.