Modifications to stock petalinux 2022.2 image for kv260

* Replace /etc/shadow with simpler one that alllows root login with blank password
* rm *.u-boot initrd file from boot
* rename and move kernel and dtb to /boot dir on ext4 partition
* move mount of vfat from /boot to /boot/u-boot
* add clean boot.scr to boot partition (will now be the only file there)