scripts/kconfig/merge_config: don't redefine 'y' to 'm'

In today's merge_config.sh the order of the config fragment files dictates
the output of a config option. With this approach we will get different
.config files depending on the order of the config fragment files.

So doing something like:
$ ./merge/kconfig/merge_config.sh selftest.config drm.config

Where selftest.config defines DRM=y and drm.config defines DRM=m, the
result will be "DRM=m".

Rework to add a switch to get builtin '=y' precedence over modules '=m',
this will result in "DRM=y". If we do something like this:

$ ./merge/kconfig/merge_config.sh -y selftest.config drm.config

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>