Add some improvements to make secure coap work on Zephyr 4.3.0
with mbedtls (not using Zephyr TLS protocol options).
We now set the build options to zephyr, so we make use of the
internal mbedtl in zephyr, so we do not require a specific header
file anymore.
In older versions of Zephyr, where mbedtls does not expose the
configuration, we can force-set the macro in the C flags, so that
mbedtls will have our required function.
This way, we do not need to change the mbedtls included in Zephyr,
while still supporting a few older versions.
Add in new functions
coap_new_client_session3()
coap_new_client_session_pki3()
coap_new_client_session_psk3()
coap_new_client_session_oscore3()
coap_new_client_session_oscore_pki3()
coap_new_client_session_oscore_psk3()
which effectively does coap_session_set_app_data2() and
coap_ws_set_host_request() early on in the session set up cycle.
New examples/zephyr directory that contains a sample coap-client.
Building the coap-client assumes that the zephyrproject has already been
installed and set up (default directory ~/zephyrproject).