configs: enable MbedTLS as default setting

Enable MbedTLS as default setting for qemu arm64 and sandbox.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 540ccef..4e33222 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -352,6 +352,7 @@
 CONFIG_FS_CRAMFS=y
 CONFIG_ADDR_MAP=y
 CONFIG_CMD_DHRYSTONE=y
+CONFIG_MBEDTLS_LIB=y
 CONFIG_ECDSA=y
 CONFIG_ECDSA_VERIFY=y
 CONFIG_TPM=y