add ci clean before build, which help with parallel compiling

Author: hathach

tools/build_all.py

@@ -35,6 +35,7 @@
     os.makedirs(bin_directory, exist_ok=True)
 
     start_time = time.monotonic()
+    subprocess.run("make -j BOARD={} clean".format(board), shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
     make_result = subprocess.run("make -j BOARD={} all".format(board), shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
     build_duration = time.monotonic() - start_time