lib/mbedtls: Switch to currently latest commit of LTS branch v2.16.

From a version numbering point of view this is a downgrade (2.17.0 ->
2.16.x).  However the latest commit for version 2.17.0 is from March 2019
and no further minor release happened after 2.17.0.  This version is EOL.
2.16.x though is still actively maintained as a long term release, hence
security and stability fixes are still being backported, including
compatibility with upcoming compiler releases.

lib/mbedtls

@@ -1 +1 @@
-Subproject commit 3f8d78411a26e833db18d9fbde0e2f0baeda87f0
+Subproject commit 1b6a24f759cefbb4a69ae9476885d7f42a847e3d