[bsp][at32]Update AT32 sdk_dist.py (#10130)

Update AT32 sdk_dist.py

fix scons --dist

Author: Dozingfiretruck

bsp/at32/tools/sdk_dist.py

@@ -28,10 +28,11 @@ def dist_do_building(BSP_ROOT, dist_dir):
     print("=> copy at32 bsp library")
     library_dir = os.path.join(dist_dir, 'libraries')
     library_path = os.path.join(os.path.dirname(BSP_ROOT), 'libraries')
-    bsp_copy_files(
-        os.path.join(library_path, rtconfig.BSP_LIBRARY_TYPE),
-        os.path.join(library_dir, rtconfig.BSP_LIBRARY_TYPE),
-    )
+    if rtconfig.BSP_LIBRARY_TYPE is not None:
+        bsp_copy_files(
+            os.path.join(library_path, rtconfig.BSP_LIBRARY_TYPE),
+            os.path.join(library_dir, rtconfig.BSP_LIBRARY_TYPE),
+        )
 
     print("=> copy bsp drivers")
     bsp_copy_files(os.path.join(library_path, 'rt_drivers'), os.path.join(library_dir, 'rt_drivers'))