kbuild: remove scripts/multiconfig.sh

We have switched to the single .config configuration system,
the same one as used in Linux Kernel.

The necessary glue code is small enough now, so move it to the
top-level Makefile and scripts/kconfig/Makefile, and then delete
scripts/multiconfig.sh.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>