scons: Put the configuration info in the build directory too.

It fixes cached configuration results from one platform being erroneously
used in other platforms.

scons/gallium.py

     env['build'] = build_dir
     env.SConsignFile(os.path.join(build_dir, '.sconsign'))
     env.CacheDir('build/cache')
+    env['CONFIGUREDIR'] = os.path.join(build_dir, 'conf')
+    env['CONFIGURELOG'] = os.path.join(os.path.abspath(build_dir), 'config.log')
 
     # Parallel build
     if env.GetOption('num_jobs') <= 1: