The source tree can contain lots of uninteresting data like tag or
cscope files, packaging which slows down make rpm needlessly. It can
also break the build, if the tree contains an unrelated file named
*.spec. The downside of this change is that new subdirectories have to
be added to the KBUILD_ALLDIRS variable in the top-level Makefile. The
upside is that the behavior is more predictable.

Signed-off-by: Michal Marek <mmarek@suse.cz>