config: init: support --aports too

We shouldn't overwrite config.aports if the user specified a custom
--aports directory. We only want to do this to update the default if
they're using a custom workdir.

Make the code here less aggressive, and correctly use the last entry in
config.aports as "pmaports" instead of the first.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>