imx: cx9020: use distro boot

We switch from custom boot commands relying on uEnv.txt to distro boot.
This removes the automatic fpga bitstream loading in favor of loading
bitstreams via custom bootscripts (boot.scr) or after booting the
kernel.

Acked-by: Patrick Bruenn <p.bruenn@beckhoff.com>
Signed-off-by: Steffen Dirkwinkel <s.dirkwinkel@beckhoff.com>