classes: files: Extend overlayfs-etc class

Add the ability to expose the lower layer of /etc when mounting overlay.
This is the similar to what overlayroot script from initramfs-framework does.

By default, this option is turned off to keep an old behavior intact.

(From OE-Core rev: 6ad25304abefcbe538db7745e17ac213fa7d0719)

Signed-off-by: Vyacheslav Yurkov <v.yurkov@precitec.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 791e8a8bacce5a7f31f4d7bcbfb17df2967fd258)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/files/overlayfs-etc-preinit.sh.in

@@ -18,6 +18,7 @@ mount -t sysfs sysfs /sys
 BASE_OVERLAY_ETC_DIR={OVERLAYFS_ETC_MOUNT_POINT}/overlay-etc
 UPPER_DIR=$BASE_OVERLAY_ETC_DIR/upper
 WORK_DIR=$BASE_OVERLAY_ETC_DIR/work
+LOWER_DIR=$BASE_OVERLAY_ETC_DIR/lower
 
 mkdir -p {OVERLAYFS_ETC_MOUNT_POINT}
 if mount -n -t {OVERLAYFS_ETC_FSTYPE} \
@@ -26,6 +27,14 @@ if mount -n -t {OVERLAYFS_ETC_FSTYPE} \
 then
     mkdir -p $UPPER_DIR
     mkdir -p $WORK_DIR
+
+    if {OVERLAYFS_ETC_EXPOSE_LOWER}; then
+        mkdir -p $LOWER_DIR
+
+        # provide read-only access to original /etc content
+        mount -o bind,ro /etc $LOWER_DIR
+    fi
+
     mount -n -t overlay \
         -o upperdir=$UPPER_DIR \
         -o lowerdir=/etc \