OpenWrt更新镜像源

国内环境，更新OpenWrt的源，经常会失败：

root@OpenWrt:~# opkg update
Downloading https://downloads.openwrt.org/snapshots/targets/ipq40xx/generic/packages/Packages.gz
*** Failed to download the package list from https://downloads.openwrt.org/snapshots/targets/ipq40xx/generic/packages/Packages.gz

Downloading https://downloads.openwrt.org/snapshots/packages/arm_cortex-a7_neon-vfpv4/base/Packages.gz
*** Failed to download the package list from https://downloads.openwrt.org/snapshots/packages/arm_cortex-a7_neon-vfpv4/base/Packages.gz

Collected errors:
 * opkg_download: Failed to download https://downloads.openwrt.org/snapshots/targets/ipq40xx/generic/packages/Packages.gz, wget returned 4.
 * opkg_download: Check your network settings and connectivity.

 * opkg_download: Failed to download https://downloads.openwrt.org/snapshots/packages/arm_cortex-a7_neon-vfpv4/base/Packages.gz, wget returned 4.
 * opkg_download: Check your network settings and connectivity.

可以考虑更换国内的镜像源，提高成功率。

默认的源，其配置文件为：

root@OpenWrt:/etc/opkg# cat distfeeds.conf
src/gz openwrt_core https://downloads.openwrt.org/snapshots/targets/ipq40xx/generic/packages
src/gz openwrt_base https://downloads.openwrt.org/snapshots/packages/arm_cortex-a7_neon-vfpv4/base

可以考虑将其替换为ustc的源，具体如下：

root@OpenWrt:/etc/opkg# cat distfeeds.conf
src/gz openwrt_core https://mirrors.ustc.edu.cn/openwrt/snapshots/targets/ipq40xx/generic/packages
src/gz openwrt_base https://mirrors.ustc.edu.cn/openwrt/snapshots/packages/arm_cortex-a7_neon-vfpv4/base

重新更新即可：

root@OpenWrt:/etc/opkg# opkg update
Downloading https://mirrors.ustc.edu.cn/openwrt/snapshots/targets/ipq40xx/generic/packages/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_core
Downloading https://mirrors.ustc.edu.cn/openwrt/snapshots/targets/ipq40xx/generic/packages/Packages.sig
Signature check passed.
Downloading https://mirrors.ustc.edu.cn/openwrt/snapshots/packages/arm_cortex-a7_neon-vfpv4/base/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_base
Downloading https://mirrors.ustc.edu.cn/openwrt/snapshots/packages/arm_cortex-a7_neon-vfpv4/base/Packages.sig
Signature check passed.