cleanup-workdir: update the way to check obsolete dirs
Update the way to check obsolete directories.
According to package and its version construct a list of all packages'
current build directory. If any directory under $WORKDIR/*/ is not in
the list will be removed.
At same time, all the files(vs. directory) under $WORKDIR and
$WORKDIR/*/ will be removed because they are not created by poky.
