Use platform name as prefix

Allows to re-use existing logic on blender side, namely the
buildbot_util.py expects platform to be a prefix.

master.cfg

@@ -147,7 +147,7 @@ def add_builder(c, name, platforms, factory, branch='',
         for slave in master_private.slaves:
             if platform == slave['platform']:
                 workernames.append(slave['name'])
-                builder_name = name + '_' + slave['platform_short']
+                builder_name = slave['platform_short'] + '_' + name
 
         if workernames:
             f = factory(builder_name, branch, rsync)