config: init: support --aports too (MR 2470)

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.

Fixes: #2386
Signed-off-by: Caleb Connolly <caleb@postmarketos.org>