core: mbedtls: fix use of SHA-256 crypto accelerated routines

The wrong name of a configuration was used to test in mbedtls if the
accelerated SHA-256 routines should be used. Fix this by using the
correct name CFG_CORE_CRYPTO_SHA256_ACCEL instead.

Fixes: 2fc5dc95a949 ("core: mbedtls: use SHA-256 crypto accelerated routines")
Reviewed-by: Jerome Forissier <jerome.forissier@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 cd4d975..516d436 100644
--- a/lib/libmbedtls/include/mbedtls_config_kernel.h
+++ b/lib/libmbedtls/include/mbedtls_config_kernel.h
@@ -31,7 +31,7 @@
 #if defined(CFG_CRYPTO_SHA224) || defined(CFG_CRYPTO_SHA256)
 #define MBEDTLS_SHA256_C
 #define MBEDTLS_MD_C
-#if defined(CFG_CRYPTO_SHA256_ACCEL)
+#if defined(CFG_CORE_CRYPTO_SHA256_ACCEL)
 #define MBEDTLS_SHA256_PROCESS_ALT
 #endif
 #endif