This ports the following barebox commit:

| commit f389531
| Author: Lucas Stach <l.stach@pengutronix.de>
| Date:   Fri Jul 22 15:00:19 2016 +0200
|
|     state: only build circular backend if MTD is enabled
|
|     The circular backend depends on MTD symbols and is only useful
|     if MTD is present. Exclude it from the build if MTD is not enabled.
|
|     Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
|     Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

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