pmbootstrap init: Wizard for new port device- and linux-packages (#821)

* pmbootstrap init: Generate new port device- and linux-package
* adds `pmbootstrap aportgen device-*` and
  `pmbootstrap aportgen linux-*`
* ask for confirmation when selecting a non-existing device
* generate the packages directly from init
* refactor aportgen code
* fixed some easy things in the linux- APKBUILD (more to come in
  follow-up PRs!)

Testing:
* Test all questions to the user from pmb.config.init and pmb.aportgen.device
  (except for the timezone question, because we would need to monkeypatch the
  os.path.exists() function, which messes up pytest, so we'd need to refactor
  the timezone function to be more testsuite friendly first)
* Run the device wizard in a testcase a few times and check the output, that
  pmbootstrap.aportgen.device and pmbootstrap.aportgen.linux create by parsing
  the resulting APKBUILDs and deviceinfo and checking its contents.
* Build the generated device package once in the same testcase

Thanks a lot to @drebrez for all the help with this one:
<https://github.com/postmarketOS/pmbootstrap/pull/821>

See also the updated porting guide:
<https://wiki.postmarketos.org/wiki/Porting_to_a_new_device>

pmb/helpers/frontend.py

@@ -80,6 +80,7 @@ def _parse_suffix(args):
 
 def aportgen(args):
     for package in args.packages:
+        logging.info("Generate aport: " + package)
         pmb.aportgen.generate(args, package)