This ports the following barebox commit:

| commit 02a0af2
| Author: Sascha Hauer <s.hauer@pengutronix.de>
| Date:   Tue May 23 09:31:29 2017 +0200
|
|     state: do not complain about missing backend-storage-type
|
|     backend-storage-type is irrelevant for mtd devices, so do not
|     complain about this option missing. The mtd backend will print
|     messages if necessary.
|
|     Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

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