Update git submodules

* Update external/boringssl from branch 'master'
  to a5c947b7c91bac52eeb5086507b67e52a59ef980
  - Revert "Revert "external/boringssl: Sync to 81080a729af568f7b5fde92b9170cc17065027c9.""
    
    This reverts commit 228bd6249d17f351ea66508b3ec3112ed1cbdf30.
    
    Reason for revert: All fixes submitted for modules affected by the ENGINE_free API change.
    
    Change-Id: I30fafafa13ec0a6390f4a9211fbf3122a8b4865f
    
diff --git a/external/boringssl b/external/boringssl
index 228bd62..a5c947b 160000
--- a/external/boringssl
+++ b/external/boringssl
@@ -1 +1 @@
-Subproject commit 228bd6249d17f351ea66508b3ec3112ed1cbdf30
+Subproject commit a5c947b7c91bac52eeb5086507b67e52a59ef980