kconfig: rename conf_split_config() to conf_touch_deps()

According to commit 2e3646e5 ("kconfig: integrate split config
into silentoldconfig"), this function was named after split-include
tool, which used to exist in old versions of Linux.

Setting aside the historical reason, rename it into a more intuitive
name. This function touches timestamp files under include/config/
in order to interact with the fixdep tool.

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