ls2080ardb: Convert to distro boot

Most new systems in U-Boot these days make use of the generic "distro"
framework which allows a user to have U-Boot scan for a bootable OS
on all available media types.

This patch extends the LS2080ARDB board to use that framework if the
hard coded NOR flash location does not contain a bootable image.

Signed-off-by: Alexander Graf <agraf@suse.de>