Autonomous random driver: create configuration option

Create a configuration option for autonomous random drivers, i.e. PSA
crypto drivers that provide a random generator, that have their own
entropy source and do not support injecting entropy from another
source.

This commit only creates the configuration option. Subsequent commits
will add the implementation and tests.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/scripts/config.py b/scripts/config.py
index ae0614a..b60f93d 100755
--- a/scripts/config.py
+++ b/scripts/config.py
@@ -185,6 +185,7 @@
     'MBEDTLS_PKCS11_C', # build dependency (libpkcs11-helper)
     'MBEDTLS_PLATFORM_NO_STD_FUNCTIONS', # removes a feature
     'MBEDTLS_PSA_CRYPTO_CONFIG', # toggles old/new style PSA config
+    'MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG', # behavior change + build dependency
     'MBEDTLS_PSA_CRYPTO_KEY_ID_ENCODES_OWNER', # incompatible with USE_PSA_CRYPTO
     'MBEDTLS_PSA_CRYPTO_SPM', # platform dependency (PSA SPM)
     'MBEDTLS_PSA_INJECT_ENTROPY', # build dependency (hook functions)