./build.py -top now ignores local build.xml files

Previous minor fixes to build.py allowed called from
anywhere in the source repository. If a build.xml is
present in the current directory, that is used. If not,
then the top-level directory is used. However, there
was no way to use the top-level when a build.xml was
present. `-top` now allows this.
Unify top-level ./build.py args

Now, "-fs", "-java", "-cpp", "-web", and "-py"
all run the builds at the proper layer, i.e.
equivalent to:

  ./build.py -f components/tools/X/build.xml