This removes the existing hack, in which we wrote the configuration to
/dev/null to trigger its evaluation. Instead a function for this
specific purpose is added.
This commit fixes the generation of autoconf headers.
The `write_autoconf` function in the RIOT wrapper was overriding
the default value of the `header` arg to None in Kconfiglib's `write_autoconf`.
It's now set to the default header.
