releasetools: Change the default key path for bvbtool

The bvb path has been changed from 'system/bvb' to 'external/bvb'

Change-Id: I8587351b7d702a287883c6bd3c9de1cd3540435b
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 9d080d9..8a5eb89 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -550,7 +550,7 @@
     cmd.extend(["--algorithm", algorithm, "--key", key_path])
   else:
     cmd.extend(["--algorithm", "SHA256_RSA4096"])
-    cmd.extend(["--key", "system/bvb/test/testkey_rsa4096.pem"])
+    cmd.extend(["--key", "external/bvb/test/testkey_rsa4096.pem"])
 
   args = info_dict.get("board_bvb_sign_boot_image_args", None)
   if args and args.strip():