Externals: Update mbedtls to 2.16.1

On a few of our buildbot instances, we get warnings about the usage of
deprecated functions. We should correct these, especially if we're
delegating to system versions of the libraries if they're available.
However, in order to do that, we need to update our library variant from
2.1.1 so that the non-deprecated alternatives are actually available.

Externals/mbedtls/include/CMakeLists.txt

@@ -0,0 +1,16 @@
+option(INSTALL_MBEDTLS_HEADERS "Install mbed TLS headers." ON)
+
+if(INSTALL_MBEDTLS_HEADERS)
+
+    file(GLOB headers "mbedtls/*.h")
+
+    install(FILES ${headers}
+        DESTINATION include/mbedtls
+        PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ)
+
+endif(INSTALL_MBEDTLS_HEADERS)
+
+# Make config.h available in an out-of-source build. ssl-opt.sh requires it.
+if (NOT ${CMAKE_CURRENT_BINARY_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
+    link_to_source(mbedtls)
+endif()