kconfig: require the argument of --defconfig

Currently, the argument for --defconfig is optional. If the argument
is not passed, the hard-coded default arch/$(ARCH)/defconfig is used.

It no longer happens in Linux since the last users of the default are
gone by the following commits:

- Commit f3e20ad6 ("s390: move arch/s390/defconfig to
  arch/s390/configs/defconfig")

- Commit 986a1376 ("alpha: move arch/alpha/defconfig to
  arch/alpha/configs/defconfig")

I want to kill the Linux-specific directory path embedded in the
Kconfig binary.

The --savedefconfig (reverse operation of --defconfig) requires an
argument, so it should not hurt to do likewise for --defconfig.

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