Use the generic "distro" boot framework to enable automatic DHCP boot.
MMC and USB are not yet implemented, so this is the only boot option.

The fdt and kernel addresses are adopted from downstream; ramdisk and
scriptaddr addresses were chosen arbitrarily.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Alexander Graf <agraf@suse.de>