Add supporting files to enable use of verbatim TF-M config

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>

configs/ext/README.md

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+Summary
+-------
+
+All files in this directory are distributed under the normal Mbed TLS dual Apache 2.0 or GPLv2-or-later
+license.
+
+Background
+-----------
+
+The two files crypto_config_profile_medium.h and tfm_mbedcrypto_config_profile_medium.h
+are taken verbatim from the TF-M source code here:
+
+https://git.trustedfirmware.org/TF-M/trusted-firmware-m.git/tree/lib/ext/mbedcrypto/mbedcrypto_config
+
+In TF-M, they are distributed under a 3-Clause BSD license, as noted at the top of the files.
+
+In Mbed TLS, with permission from the TF-M project, they are distributed under a dual [Apache-2.0](https://spdx.org/licenses/Apache-2.0.html) OR [GPL-2.0-or-later](https://spdx.org/licenses/GPL-2.0-or-later.html) license.
+
+We only retain the note at the top of the files because we are taking the files un-modified, for ease of
+maintenance.

configs/ext/config_tfm.h

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+/*
+ *  Empty placeholder
+ *
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ */
+
+/*
+ * This file is intentionally empty.
+ *
+ * Having an empty file here allows us to build the TF-M config, which references this file,
+ * without making any changes to the TF-M config.
+ */

configs/ext/mbedtls_entropy_nv_seed_config.h

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+/*
+ *  Empty placeholder
+ *
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ */
+
+/*
+ * This file is intentionally empty.
+ *
+ * Having an empty file here allows us to build the TF-M config, which references this file,
+ * without making any changes to the TF-M config.
+ */