aliBuild clean should be able to clean broken builds

[ktf]

If a build is broken, we should have a way to garbage collect it from sw/BUILD. Currently this is not done because the "latest" symlink is there, to simplify debugging.

[ktf]

@Barthelemy

[Barthelemy]

This is still something that would be useful. It happens to have something broken and no way to recover without doing "rm" of each folder with the component in its name.

[Barthelemy]

This is still something I would like to see (soonish).

[Barthelemy]

Again and again I end up in situations where I can't easily clean broken builds. We need a "clean" command. This time, out of desperation, I deleted "sw" and destroyed my installation :) see #428

[syamasak]

It is nice to have alibuild full-clean which just does
rm -rf sw/{BUILD,INSTALLROOT,TARS,osx_x86-64}
This is the faster and easier way to clean broken builds.