volatile-binds: add recipe

This recipe is designed to play a key role in a read-only rootfs
of systemd based systems. It generates service files from a template,
volatile-binds.service.in and the VOLATILE_BINDS variable.

By default, VOLATILE_BINDS takes the value of "/var/volatile/lib /var/lib\n",
which leads to the generation of volatile-var-lib.service file.
This file doesn't have any effect in a read-write system, as it
has "ConditionPathIsReadWrite = !/var/lib" in the [Unit] section.
In other words, this file only has effect in a read-only rootfs.

(From OE-Core rev: ed7d30dc0cdb6d6c56c50ac7a3440c4ed0ee70d3)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-core/volatile-binds/files/mount-copybind

@@ -0,0 +1,34 @@
+#!/bin/sh
+#
+# Perform a bind mount, copying existing files as we do so to ensure the
+# overlaid path has the necessary content.
+
+if [ $# -lt 2 ]; then
+    echo >&2 "Usage: $0 spec mountpoint [OPTIONS]"
+    exit 1
+fi
+
+spec=$1
+mountpoint=$2
+
+if [ $# -gt 2 ]; then
+    options=$3
+else
+    options=
+fi
+
+[ -n "$options" ] && options=",$options"
+
+mkdir -p "${spec%/*}"
+if [ -d "$mountpoint" ]; then
+    if [ ! -d "$spec" ]; then
+        mkdir "$spec"
+        cp -pPR "$mountpoint"/. "$spec/"
+    fi
+elif [ -f "$mountpoint" ]; then
+    if [ ! -f "$spec" ]; then
+        cp -pP "$mountpoint" "$spec"
+    fi
+fi
+
+mount -o "bind$options" "$spec" "$mountpoint"