Add property build_platform

To make easier for us to remove the hardcoded entries that we have in our build system

Author: opacam

pythonforandroid/util.py

@@ -1,6 +1,6 @@
 import contextlib
 from os.path import exists, join
-from os import getcwd, chdir, makedirs, walk
+from os import getcwd, chdir, makedirs, walk, uname
 import io
 import json
 import shutil
@@ -24,6 +24,13 @@ class WgetDownloader(FancyURLopener):
 urlretrieve = WgetDownloader().retrieve
 
 
+build_platform = '{system}-{machine}'.format(
+    system=uname()[0], machine=uname()[-1]).lower()
+"""the build platform in the format `system-machine`. We use
+this string to define the right build system when compiling some recipes or
+to get the right path for clang compiler"""
+
+
 @contextlib.contextmanager
 def current_directory(new_dir):
     cur_dir = getcwd()