Add configurability via devicetree ...

This adds the possibility to configure the place for the environment
from the devicetree and to partition devices from the devicetree.

Configuration has the general form of devices with a regular compatible
property. This allows to later add additional drivers or drivers with
different behaviour (for example to add support for redundant environment).

The configuration is all in the /chosen/barebox/ hierarchy of the
devicetree. This separates the configuration from the hardware
description. Also it makes it possible to store the configuration
in a completely separate devicetree (or devicetree overlay). For
the same reason all configuration is done using nodepathes rather
than phandles.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>