Build ubuntu image for rockchip rk35xx machines

Instructions:

1. Create rootfs

   For example:

   sudo ./mkrootfs.sh focal

   sudo ./mkrootfs.sh jammy-xfce

   You can run it multiple times. If you think there is a problem with the created rootfs, you can clear it:

   sudo ./mkrootfs.sh focal clean

   sudo ./mkrootfs.sh jammy-xfce clean

2. Create target image

   For example:

   sudo ./mkimg.sh rk3568 h68k focal

   sudo ./mkimg.sh rk3568 h69k-max jammy-xfce

3. Get the target image:

   build/h68k_ubuntu_focal_vYYYYMMDD.img

   build/h69k-max_ubuntu_jammy-xfce_vYYYYMMDD.img

4. System Requirements：

   x86_64 host: debian or ubuntu

   arm64 host: debian or ubuntu or armbian

   Storage space: >= 8GB