ARM: am335x: Add a NAND update handler for the regular barebox
To be able to not only update the MLO in NAND but also the
regular barebox image.
Since this is implemented with help of the corresponding xload
handler this also removes the 'xload' from the Kconfig options
and the filename.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>