New ./configure option
--with-mbedtls-oscore
If --disable-dtls is set, then MbedTLS is used as the OSCORE backend, otherwise
the MbedTLS library is used for both (D)TLS and OSCORE.
New cmake option
-DOSCORE_BACKEND=mbedtls
If -DENABLE_DTLS=OFF is set, then MbedTLS is used as the OSCORE backend, otherwise
the MbedTLS library is used for both (D)TLS and OSCORE.
New Zephyr option
config MBEDTLS_OSCORE
If set to y and CONFIG_LIBCOAP_OSCORE_SUPPORT is set to y, then the Zephyr MbedTLS
backend will get used for the OSCORE backend (no (D)TLS).