SCons: fix the build.

My last SCons commit broke the build because I messed up the value of charms_yml_in_build,
so that it was always empty, so nothing else built.
Scons: more aliases, better cleaning.

- Clean empty dirs when cleaning all.
- Properly add alias for drac.
- Add aliases for "all" and "charms".
- Make "pdf" same as "book", since I keep wanting to type that.
- Add emitters to track the extra outputs for pdf, html and stats,
so that they get cleaned properly.