Makefile: change dependency to prevent needless builds

Commit

   3dabecf4 Makefile: bugfix for parallel builds

forced all object files to be rebuilt every time, while fiing
MAKEALL errors.  By making objects depend on .config instead of
depending on silentoldconfig we force a rebuild only when the
configuration changed.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>