arm: kirkwood: lsxl: add note about sata in README

The boot commands have changed in the environment. Add a note about the
incompatible change and how resolve the issue in the board's README.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>