Pass barebox-detected bootsource to Linux to make it availible to
Linux userspace. That information is passed as full path to the node
corresponding to the bootsource and is placed under /chosen/bootsource
and it can be read under Linux in

/sys/firmware/devicetree/base/chosen/bootsource

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>