Makefile: Ensure CVS directories are removed from target fs.

The find command syntax is incorrect when deleting CVS and .svn directories
from the target filesystem, which prevent CVS directories from being
deleted.

It's useful to delete these dirs because users of buildroot often import
buildroot into CVS or Subversion.

Signed-off-by: Will Newton <will.newton@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>