WIP: Add support for a separate aports directory & repository.

Created by: marmistrz

Closes #383 (closed). Contains changes from #1454 (to avoid later rebasing)

NB, If Python 3.6 were supported, a much cleaner PEP-498 could be used:

        logging.fatal(f"    git clone {APORTS_GIT} {args.aports}")

---

[x] Merge on GitHub (see https://postmarketos.org/merge)

.travis/travis_script.sh

@@ -23,6 +23,8 @@ test/static_code_analysis.sh
 fold_end "static_analysis"
 
 # pmbootstrap init
+mkdir -p ~/.cache/pmbootstrap
+ln -s aports ~/.cache/pmbootstrap/
 fold_start "init" "pmbootstrap init"
 yes "" | ./pmbootstrap.py init
 fold_end "init"