Move challenge code to own folder (#64)

...and add a stub for 'pmbootstrap challenge APKINDEX.tar.gz'.

pmb/challenge/frontend.py

@@ -0,0 +1,31 @@
+"""
+Copyright 2017 Oliver Smith
+
+This file is part of pmbootstrap.
+
+pmbootstrap is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+pmbootstrap is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with pmbootstrap.  If not, see <http://www.gnu.org/licenses/>.
+"""
+import os
+import pmb.challenge
+
+
+def frontend(args):
+    path = args.challenge_file
+    if path.endswith(".apk"):
+        pmb.challenge.build(args, path)
+    elif os.path.basename(path) == "APKINDEX.tar.gz":
+        pmb.challenge.apkindex(args, path)
+    else:
+        raise ValueError("It is only possible to challenge files ending"
+                         " in .apk or files named APKINDEX.tar.gz.")