Caching pointers to device tree nodes or names is not safe. The barebox
internal device tree may be changed by loading a new device tree or through
fixup handlers. As a result, the string may be deleted.
Use local copies of the full path instead.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>