Enabling MBEDTLS_MEMORY_BUFFER_ALLOC_C module together with
MBEDTLS_PLATFORM_MEMORY causes the library to use its own malloc
replacement. This makes memory management analyzers such as ASan
largely ineffective. We now test MBEDTLS_MEMORY_BUFFER_ALLOC_C
separately. Disable it in the "full" config.
This mirrors a change that was made in Mbed TLS on config.pl and had
not been ported to Mbed Crypto yet.
With this commit, config.py is aligned in Mbed Crypto and Mbed TLS.
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