distributions对于 Ubuntu 仓库：
Origin My
Label: test-xenial
Codename: xenial
Architectures: armhf
Components: main
Description: rep to test
SignWith: 7B5AAC2D
Update: upstream-update-xenial
updates：
Name: upstream-update-xenial
Method: http://ports.ubuntu.com/ubuntu-ports
VerifyRelease: blindtrust
Components: main
Architectures: armhf
FilterList: purge ../mirror.packages
mirror.packages有这一行：
libc6-dev:armhf install
命令：
reprepro --keepunreferencedfiles --noskipold -b "$localrepo" update xenial
但libc6-dev尚未下载。我遗漏了什么？