Buildroot for Loongson Boards

Collect open source port of Buildroot to Loongson Boards by Loongson:

1. Loongson port based on Buildroot 2021.02.4 in branch loongson-2021.02.4 found via:
   1. Access https://github.com/loongarch64/projX-2k500-kernel, find the baiduyun download link at https://pan.baidu.com/s/1CfAeV3mSDJPw6zyw5Mv0ww (password is nl80).
   2. Locate the tarball at 广东龙芯2K500-先锋板-v1.0/04-文件系统/源码/buildroot-2021.src.b5c2bd4-build.20230718090721.tar.gz