makebuildserver: cache defaults to ~/.cache/fdroidserver

This also provides a config option to override that default.  ~/.cache is
a standard location on GNU/Linux machines for cached content.  It is also
good to have the cache outside of the git repo in case `git clean -fdx` is
run, which would delete all files in the directory that are not part of the
git repo, including buildserver/cache/

makebuildserver

@@ -50,6 +50,7 @@ config = {
     'arch64': False,
     'basebox': 'jessie32',
     'baseboxurl': 'https://f-droid.org/jessie32.box',
+    'cachedir': os.path.join(os.getenv('HOME'), '.cache', 'fdroidserver'),
     'cpus': 1,
     'memory': 3584,
 }
@@ -72,9 +73,9 @@ if options.clean:
     vagrant(['destroy', '-f'], cwd=serverdir, printout=options.verbose)
 
 # Update cached files.
-cachedir = os.path.join('buildserver', 'cache')
+cachedir = config['cachedir']
 if not os.path.exists(cachedir):
-    os.mkdir(cachedir)
+    os.makedirs(cachedir, 0755)
 
 cachefiles = [
     ('android-sdk_r24.4.1-linux.tgz',