kbuild: fix warning "overriding recipe for target 'Makefile'"

If you do "make Makefile" with GNU Make 3.x, the following warning
is displayed:

  $ make Makefile
  Makefile:165: warning: overriding recipe for target 'Makefile'
  Makefile:51: warning: ignoring old recipe for target 'Makefile'
  make[1]: Nothing to be done for 'Makefile'.
  make: Nothing to be done for 'Makefile'.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>