The clean-dirs target does not only clean the repos present in
PROJECTS, but all repos it finds to be dirty, and clean-all-dirs does
the opposite. I suppose that was an oversight, swap their recipes.
Moreover, cleaning all directories goes about its business in an
overly complicated and unecessarily time-consuming way, fix that,
too.
Signed-off-by: Jan Lindemann <jan@janware.com>