How stupid I am… I could do this months ago. Just remove the folder named bin, staging_dir, build_dir, dl, then compress, the source code is small enough to download.
1. install Ubuntu 14.04 64bit Server to Virtual Machine.
2. in virtualbox, we will need install some packages to compile openwrt:
sudo apt-get install gcc g++ binutils patch bzip2 flex bison make autoconf gettext texinfo unzip sharutils subversion libncurses5-dev ncurses-term zlib1g-dev libssl-dev python
3. download openwrt at vonger.cn/misc/vocore2/openwrt.tar.bz2
4. call “tar -jxvf openwrt.tar.bz2” to uncompress the package.
5. call “make menuconfig” in the openwrt folder, configure openwrt in menu, choose MT7688/28/VoCore2 in menu.
6. call “make” in the openwrt folder, start the make process
PS: recently I have no word about the lame post carrier…