use a git submodule for the beaker source tree

This way we can be sure the correct branch is always checked out and up
to date when building the web site.

During development it is still possible to pass a path to a different
beaker working copy if needed, for example:

    make BEAKER=../beaker man

Change-Id: Ibae794b1b9d5c675651c6cd50fae0ae561a9ba56