libmbedtls: configure and compile mbedtls

Configures mbedtls with a minimal user mode TA configuration and makes
it compile.

Adds dummy include/mbedtls_config_kernel.h to give a good error message
in case mbedTLS is compiled in for kernel mode.

mbedTLS is enabled for TAs with CFG_TA_MBEDTLS = y
Builtin self tests are enabled with CFG_TA_MBEDTLS_SELF_TEST = y

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
new file mode 100644
index 0000000..771795c
--- /dev/null
+++ b/lib/libmbedtls/include/mbedtls_config_kernel.h
@@ -0,0 +1,3 @@
+/* SPDX-License-Identifier: BSD-2-Clause */
+/* Copyright (c) 2018, Linaro Limited */
+#error "mbedTLS is not yet supported in kernel mode"