Adapt buffer size for minimal CCM config
Adapt buffer size for minimal CCM config

This is useful for testing interop with GnuTLS, which sends records larger
than 512 bytes.

This change is triggered by the addition of CCM interop testing with GnuTLS a
few commits ago.

NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls