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* Consider APKINDEX files older than 4 hours as outdated and download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/file.py

@@ -18,7 +18,7 @@ along with pmbootstrap.  If not, see <http://www.gnu.org/licenses/>.
 """
 
 import os
-
+import time
 import pmb.helpers.run
 
 
@@ -60,6 +60,16 @@ def is_up_to_date(path_sources, path_target=None, lastmod_target=None):
     return lastmod_target >= lastmod_source
 
 
+def is_older_than(path, seconds):
+    """
+    Check if a single file is older than a given amount of seconds.
+    """
+    if not os.path.exists(path):
+        return True
+    lastmod = os.path.getmtime(path)
+    return lastmod + seconds < time.time()
+
+
 def symlink(args, file, link):
     """
     Checks if the symlink is already present, otherwise create it.