Introduce the new build time option '--enable-state-backward-compatibility' to
port the following barebox commit.

NOTE: This changes barebox-state's default behaviour.

| commit 480cde1
| Author: Juergen Borleis <jbe@pengutronix.de>
| Date:   Tue Aug 15 15:46:31 2017 +0200
|
|     state: keep backward compatibility
|
|     Previous 'state' variable set variants do not know and use metadata. The
|     'direct' storage backend's read function honors this, but not its
|     counterpart the write function. This makes an update of the 'state'
|     variable set impossible.
|     This change makes backward compatibility explicit, else it complains in
|     the read function as well. With some more debug output it helps the
|     developer to do things right.
|
|     Signed-off-by: Juergen Borleis <jbe@pengutronix.de>
|     Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Signed-off-by: Ulrich Ölmann <u.oelmann@pengutronix.de>