* [ general ] Change Toolchain download method. Introduce torrent downloads by using Aria2. If torrent download fails it proceeds to download from servers. It can download from all mirrors at once, so download speed is dramatically increased. @mzhboy DOWNLOAD_MIRROR parameter is not needed here anymore. The same method is used to download rootfilesystem cache. Based on ideas from here https://github.com/armbian/build/pull/1364
* Added yet another mirror for speeding up the download process, few small adjustements for torrent downloading
* Optimising direct download when DOWNLOAD_MIRROR is set to 'china' due to aria2 download limitations
* Both mirrors support HTTPS, changing