build_component.py should not write over populated target directories

[aaaaalbert]

Currently, build_component.py removes the contents of the build target directory before copying over files anew. This can hurt if you happen to specify the wrong target dir, or have files you created / edited in the target dir but did not commit them yet.

Since it's trivial to clean / recreate the target dir before you build yourself, I think a good thing to do is to require the person starting the build perform these steps consciously. build_component should only complain if the target dir is not empty, but not attempt to clean it.

@XuefengHuang provided a patch to the (outdated) preparetest script providing that functionality, see SeattleTestbed/dist#143.

[aaaaalbert]

Note: Just like rm -rf *, build_component does not remove "hidden" files such as .FILENAME on Unixoid systems.

[aaaaalbert]

We have b9cd955, but no unit tests and no pull request to go with it. Please lend a hand!