Explain how PSA_CRYPTO_DRIVER_TEST_ALL works and why we have it. Note that
it is incompatible with MBEDTLS_PSA_CRYPTO_CONFIG. As a consequence, disable
that option in component_test_psa_crypto_drivers.
MBEDTLS_PSA_CRYPTO_BUILTIN_KEYS is in the full config, so there's no need to
add it explicitly.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>