Change download images compression format

We decided to go with xz

• our system creates 7z or gz https://github.com/armbian/build/blob/master/lib/debootstrap.sh#L657

• download links are without extensions https://dl.armbian.com/nanopi-r2s/Bionic_current which are symlinked to the whatever file extension ... ready there

How to provide md5 and shasum files?