ext: lib: mbedtls: Upgrading mbedTLS library

Upgrading mbedTLS to version 2.4.2 from 2.4

Origin: https://tls.mbed.org/download/start/mbedtls-2.4.2-apache.tgz



Jira: ZEP-1800

Change-Id: I16a7eaeb4c2e47d11f0594fe1bd865be3eef37b6
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
(cherry picked from commit a20b2495)