This ports the following barebox commit:

| commit 09f15cd
| Author: Daniel Schultz <d.schultz@phytec.de>
| Date:   Fri Nov 3 11:48:28 2017 +0100
|
|     common: state: Add function to read state MAC
|
|     This API function allows to receive a copy of a MAC address from
|     variables in a state.
|
|     Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
|     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>