core: mbedtls: use SHA-256 crypto accelerated routines

Uses the recently provided accelerated SHA-256 routine.

Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/lib/libmbedtls/include/mbedtls_config_kernel.h b/lib/libmbedtls/include/mbedtls_config_kernel.h
index 9b68e4c..826850c 100644
--- a/lib/libmbedtls/include/mbedtls_config_kernel.h
+++ b/lib/libmbedtls/include/mbedtls_config_kernel.h
@@ -31,6 +31,9 @@
 #if defined(CFG_CRYPTO_SHA224) || defined(CFG_CRYPTO_SHA256)
 #define MBEDTLS_SHA256_C
 #define MBEDTLS_MD_C
+#if defined(CFG_CRYPTO_SHA256_ACCEL)
+#define MBEDTLS_SHA256_PROCESS_ALT
+#endif
 #endif
 
 #if defined(CFG_CRYPTO_SHA384) || defined(CFG_CRYPTO_SHA512)