树莓派4 8GB版本安装ubuntu20

实测ubuntu18 没有办法在树莓派4 8GB运行，原因不明。

会出现如下错误：

recover4.elf not found
recovery.elf not found
start4.elf not found

安装ubuntu20可以正常进入系统

所以这就成了树莓派4 8GB版本安装ubuntu20教程

首先换软件源（清华源）

deb http://mirrors.aliyun.com/ubuntu-ports focal main restricted
deb http://mirrors.aliyun.com/ubuntu-ports focal-updates main restricted
deb http://mirrors.aliyun.com/ubuntu-ports focal universe
deb http://mirrors.aliyun.com/ubuntu-ports focal-updates universe
deb http://mirrors.aliyun.com/ubuntu-ports focal multiverse
deb http://mirrors.aliyun.com/ubuntu-ports focal-updates multiverse
deb http://mirrors.aliyun.com/ubuntu-ports focal-backports main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu-ports focal-security main restricted
deb http://mirrors.aliyun.com/ubuntu-ports focal-security universe
deb http://mirrors.aliyun.com/ubuntu-ports focal-security multiverse

deb-src http://mirrors.aliyun.com/ubuntu-ports focal main restricted
deb-src http://mirrors.aliyun.com/ubuntu-ports focal-updates main restricted
deb-src http://mirrors.aliyun.com/ubuntu-ports focal universe
deb-src http://mirrors.aliyun.com/ubuntu-ports focal-updates universe
deb-src http://mirrors.aliyun.com/ubuntu-ports focal multiverse
deb-src http://mirrors.aliyun.com/ubuntu-ports focal-updates multiverse
deb-src http://mirrors.aliyun.com/ubuntu-ports focal-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu-ports focal-security main restricted
deb-src http://mirrors.aliyun.com/ubuntu-ports focal-security universe
deb-src http://mirrors.aliyun.com/ubuntu-ports focal-security multiverse