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

tools/tools/nanobsd/defaults.sh

@@ -724,6 +724,50 @@ populate_slice() {
 	nano_umount ${mnt}
 }
 
+_populate_part() (
+	local dir fs lbl metalog size type
+	type=$1
+	fs=$2
+	dir=$3
+	lbl=$4
+	size=$5
+	metalog=$6
+
+	echo "Creating ${fs}"
+
+	# Use the directory provided, otherwise create an empty one temporarily.
+	if [ -n "${dir}" ] && [ -d "${dir}" ]; then
+		echo "Populating ${lbl} from ${dir}"
+	else
+		if [ "${type}" = "cfg" ]; then
+			dir=$(mktemp -d -p "${NANO_OBJ}" -t "${type}")
+			trap "rm -f ${dir}" 1 2 15 EXIT
+		fi
+	fi
+
+	if [ -d "${dir}" ]; then
+		# If there is no metalog, create one using the default
+		# NANO_DEF_UNAME and NANO_DEF_GNAME for all entries in the spec.
+		if [ -z "${metalog}" ]; then
+			metalog=$(mktemp -p "${NANO_OBJ}" -t "${type}")
+			trap "rm -f ${metalog}" 1 2 15 EXIT
+			echo "/set type=dir uname=${NANO_DEF_UNAME}" \
+			    "gname=${NANO_DEF_GNAME} mode=0755" > "${metalog}"
+			echo ". type=dir uname=${NANO_DEF_UNAME}" \
+			    "gname=${NANO_DEF_GNAME} mode=0755" >> "${metalog}"
+			(
+				cd "${dir}"
+				mtree -bc -k flags,gid,gname,link,mode,uid,uname |
+				    mtree -C | tail -n +2 |
+				    sed 's/uid=[[:digit:]]*/uname=root/g' |
+				    sed 's/gid=[[:digit:]]*/gname=wheel/g' >> "${metalog}"
+			)
+		fi
+
+		nano_makefs "-DxZ ${NANO_MAKEFS}" "${metalog}" "${size}" "${fs}" "${dir}"
+	fi
+)
+
 populate_cfg_slice() {
 	populate_slice "$1" "$2" "$3" "$4"
 }