zynq: Enable distro boot

Distro boot allows devices to boot using standardized boot methods by
default. This can be very handy for distributions that want to run on
different platforms.

This patch moves the zynq platform to use its old, zynq specific boot
method first and then fall back to distro boot. That way supporting
Linux distributions like openSUSE is much easier.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>