[general] new option DOWNLOAD_MIRROR to select download mirror

current DOWNLOAD_MIRROR=china is used
the mirror is provided by tsinghua university

lib/configuration.sh

@@ -244,6 +244,11 @@ esac
 DEBIAN_MIRROR='httpredir.debian.org/debian'
 UBUNTU_MIRROR='ports.ubuntu.com/'
 
+if [[ $DOWNLOAD_MIRROR == china ]] ; then
+	DEBIAN_MIRROR='mirrors.tuna.tsinghua.edu.cn/debian'
+	UBUNTU_MIRROR='mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/'
+fi
+
 # For user override
 if [[ -f $SRC/userpatches/lib.config ]]; then
 	display_alert "Using user configuration override" "userpatches/lib.config" "info"

lib/general.sh

@@ -662,6 +662,9 @@ prepare_host()
 	# Use backup server by default to balance the load
 
 	ARMBIANSERVER=dl.armbian.com
+	if [[ $DOWNLOAD_MIRROR == 'china' ]]; then
+		ARMBIANSERVER='mirrors.tuna.tsinghua.edu.cn/armbian-releases'
+	fi
 
 	local toolchains=(
 		"https://${ARMBIANSERVER}/_toolchains/gcc-linaro-arm-linux-gnueabihf-4.8-2014.04_linux.tar.xz"