feat(auth): enable MBEDTLS_CHECK_RETURN_WARNING

Define the MBEDTLS_CHECK_RETURN_WARNING macro in mbedTLS configuration
file to get compile-time warnings for mbedTLS functions we call and do
not check the return value of. Right now, this does not flag anything
but it could help catching bugs in the future.

This was a new feature introduced in mbed TLS 2.28.0 release.

Change-Id: If26f3c83b6ccc8bc60e75c3e582ab20817d047aa
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>