This change fixes two bugs:
1) If you run `serenity.sh` outside of your serenity git clone, the
`get_top_dir()` function won't be able to auto detect the serenity
root dir and we'll error out. By allowing the script to use an
existing $SERENITY_ROOT vlaue if it exists, we can solve
this problem.
2) If tried to run a command which ends up calling `build_toolchain()`
and you were outside of the root of the project, it should just
fail. Fix this by utilizing `$SERENITY_ROOT`.
253114e3b1
