Gate entropy injection through a dedicated configuration option

Entropy injection has specific testing requirements. Therefore it
should depend on a specific option.
diff --git a/scripts/config.pl b/scripts/config.pl
index fc71f65..6927c4b 100755
--- a/scripts/config.pl
+++ b/scripts/config.pl
@@ -102,6 +102,7 @@
 MBEDTLS_NO_64BIT_MULTIPLICATION
 MBEDTLS_PSA_CRYPTO_SPM
 MBEDTLS_PSA_CRYPTO_KEY_FILE_ID_ENCODES_OWNER
+MBEDTLS_PSA_INJECT_ENTROPY
 MBEDTLS_USE_PSA_CRYPTO
 _ALT\s*$
 );