We need to collect a list of all modules during the recursive build. I used
a "touch .tmp_versions/<path/to/module.ko>" to do so, which however doesn't
work so well, when path/to isn't inside the kernel tree.
  
The best way to build external modules is currently using kbuild by saying
"make SUBDIRS=/some/external/dir modules", which was thus broken. While this
way is not all that optimal and I hope to come up with something better
before 2.6, it works and should keep working, so this patch fixes the usage
above.
  
Instead of touching files with the entire path added, we just create a
<module>.mod file in $(MODVERDIR) now, and save the path to the module.ko 
in it. Since module names are unique, a flat hierarchy is actually fine here.