initscripts: add read-only-rootfs-hook.sh script

Add read-only-rootfs-hook.sh script to support a read-only rootfs.
This script makes a union mount of /var/lib and /var/volatile/lib,
making /var/lib directory writable.

[YOCTO #3406]

(From OE-Core rev: a9591158962eee1f8ae04168d6256032ecd7bc6b)

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/initscripts/initscripts-1.0/read-only-rootfs-hook.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+
+. /etc/default/rcS
+
+[ "$ROOTFS_READ_ONLY" = "no" ] && exit 0
+
+# Make sure unionfs is in /proc/filesystems
+if ! grep -q unionfs /proc/filesystems; then
+    echo "ERROR: unionfs not supported by kernel!"
+    exit 1
+fi
+
+mkdir -p /var/volatile/lib
+mount -t unionfs -o dirs=/var/volatile/lib:/var/lib=ro none /var/lib
+
+if [ $? != 0 ]; then
+    echo "ERROR: Union mount failed!"
+    exit 1
+fi