Explicitly enable built-in entropy in sample and test configs

Now that built-in entropy is a positive option
`MBEDTLS_PSA_BUILTIN_GET_ENTROPY` instead of a negative option
`MBEDTLS_NO_DEFAULT_ENTROPY_SOURCES`, it needs to be enabled explicitly in
sample and test configurations.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/configs/crypto-config-thread.h b/configs/crypto-config-thread.h
index 18206e1..5475a0a 100644
--- a/configs/crypto-config-thread.h
+++ b/configs/crypto-config-thread.h
@@ -60,6 +60,7 @@
 #define MBEDTLS_MD_C
 #define MBEDTLS_PK_C
 #define MBEDTLS_PK_PARSE_C
+#define MBEDTLS_PSA_BUILTIN_GET_ENTROPY
 
 /* Save RAM at the expense of ROM */
 #define MBEDTLS_AES_ROM_TABLES