Close #871: Enable binary repository (#887)

* add my own build key
* enable the repo in the config
* update the README file
* Adjust testcase, that validates the keys and enable it in testcases_fast.sh
* Only save/load keys to/from the config file, which we ask for during
  'pmbootstrap init', so the binary repo gets used even if a config file
  already exists (this also removes a workaround, that deletes the work
  folder path from the config dictionary before writing it)
* Download missing APKINDEX.tar.gz files with Python code, before
  attempting to build packages (so we know which ones aleady exist in
  the binary packages repository)
* Consider APKINDEX files older than 4 hours as outdated and download
  them again (also in Python code)
* Provide 'pmbootstrap update' to force-update the APKINDEX files
* Travis: more logging output on failure
* Only allow keys from config_keys to be used by "pmbootstrap config"

pmb/helpers/http.py

@@ -24,7 +24,7 @@ import urllib.request
 import pmb.helpers.run
 
 
-def download(args, url, prefix, cache=True):
+def download(args, url, prefix, cache=True, loglevel=logging.INFO):
     """
     Download a file to disk.
     """
@@ -42,7 +42,7 @@ def download(args, url, prefix, cache=True):
         pmb.helpers.run.user(args, ["rm", path])
 
     # Download the file
-    logging.info("Download " + url)
+    logging.log(loglevel, "Download " + url)
     with urllib.request.urlopen(url) as response:
         with open(path, "wb") as handle:
             shutil.copyfileobj(response, handle)