Add --no-build flag support in make_webapp.py
Add --no-build flag support in make_webapp.py

When --no-build flag specified, the make_webapp.py script will only checkout
webapps with needed patch files patched.
The requirement is requested by buildbot team which only need workable demo
source code.

crosswalk-project/crosswalk-demos,crosswalk-project/crosswalk-demos,zqzhang/crosswalk-demos,hgl888/crosswalk-demos,zqzhang/crosswalk-demos,hgl888/crosswalk-demos