common: print cd pdw on paths

print only modified variables on PATH

use common.run_cmd everywhere to get full bash bash commands

readme: recommend private/ instead of the cryptic p/

build-crosstool-ng

@@ -70,7 +70,7 @@ def main(args, extra_args=None):
         ],
         out_file=os.path.join(common.crosstool_ng_build_dir, 'lkmc.log'),
         delete_env=['LD_LIBRARY_PATH'],
-        extra_env={'PATH': common.ccache_dir + ':' + os.environ['PATH']},
+        extra_paths=[common.ccache_dir],
     ) == 0
 
 if __name__ == '__main__':