kconfig: pass new conf_changed value to the callback

Commit ee06a3ef ("kconfig: Update config changed flag before calling
callback") pointed out that conf_updated flag must be updated _before_
calling the callback, which needs to know the new value.

Given that, it makes sense to directly pass the new value to the
callback.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>