based on: http://dev.chromium.org/chromium-os/building-chromium-os/build-instructions
user# mkdir google_src; cd google_src;
user# gclient config http://src.chromium.org/git/chromiumos.git
(got some error 128, but re-sync again)
user# gclient sync -v
user# ln -s ~/google_src/chromiumos/chromiumos.git ~/chromiumos
user# cd ~/chromiumos/src/scripts
user# ./make_chroot.sh
follow URL, Method 2.
time ./build_kernel.sh
real 23m55.307s
user 32m27.170s
sys 4m16.988s
time ./build_image.sh
real 7m25.654s
user 1m18.813s
sys 0m58.612s