Add mbedtls_ssl_is_handshake_over() function

Add function to query if SSL handshake is over or not, in order to
determine when to stop calling mbedtls_ssl_handshake_step among other
things. Document function, and add warnings that the previous method of
ascertaining if handshake was over is now deprecated, and may break in
future releases.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/ChangeLog.d/add_handshake_completion_accessor b/ChangeLog.d/add_handshake_completion_accessor
new file mode 100644
index 0000000..e2b28cf
--- /dev/null
+++ b/ChangeLog.d/add_handshake_completion_accessor
@@ -0,0 +1,4 @@
+Features
+   * Add function mbedtls_ssl_is_handshake_over() to enable querying if the SSL
+     Handshake has completed or not, and thus whether to continue calling
+     mbedtls_ssl_handshake_step(), requested in #4383