结论：

执行如下命令

pacman-key --populate archlinuxarm

猫盘安装好，准备更新软件包

pacman -Syu

却的得到如下错误信息：

:: Import PGP key 77193F152BDBE6A6, "Arch Linux ARM Build System <builder+seattle@archlinuxarm.org>"? [Y/n] y
error: key "77193F152BDBE6A6" could not be looked up remotely
error: required key missing from keyring
error: failed to commit transaction (unexpected error)
Errors occurred, no packages were upgraded.

于是百度查到

pacman-key --init

无果，还有的说修改 /etc/pacman.conf 的 SigLevel = Never，也不好使，可是在修改 /etc/pacman.conf 时在这个文件里发现一些注释：

# NOTE: You must run `pacman-key --init` before first using pacman; the local
# keyring can then be populated with the keys of all official Arch Linux ARM
# packagers with `pacman-key --populate archlinuxarm`.

于是尝试

pacman-key --populate archlinuxarm

就行了，然后痛快的更新软件包

pacman -Syu