Added submodule contents into tree

externals/mbedtls/programs/test/cmake_subproject/.gitignore

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+build
+Makefile
+cmake_subproject

externals/mbedtls/programs/test/cmake_subproject/CMakeLists.txt

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+cmake_minimum_required(VERSION 3.5.1)
+
+# Test the target renaming support by adding a prefix to the targets built
+set(MBEDTLS_TARGET_PREFIX subproject_test_)
+
+# We use the parent Mbed TLS directory as the MBEDTLS_DIR for this test. Other
+# projects that use Mbed TLS as a subproject are likely to add by their own
+# relative paths.
+set(MBEDTLS_DIR ../../../)
+
+# Add Mbed TLS as a subdirectory.
+add_subdirectory(${MBEDTLS_DIR} build)
+
+# Link against all the Mbed TLS libraries. Verifies that the targets have been
+# created using the specified prefix
+set(libs
+    subproject_test_mbedcrypto
+    subproject_test_mbedx509
+    subproject_test_mbedtls
+)
+
+add_executable(cmake_subproject cmake_subproject.c)
+target_link_libraries(cmake_subproject ${libs} ${CMAKE_THREAD_LIBS_INIT})

externals/mbedtls/programs/test/cmake_subproject/cmake_subproject.c

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+/*
+ *  Simple program to test that CMake builds with Mbed TLS as a subdirectory
+ *  work correctly.
+ *
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ */
+
+#include "mbedtls/build_info.h"
+
+#include "mbedtls/platform.h"
+
+#include "mbedtls/version.h"
+
+/* The main reason to build this is for testing the CMake build, so the program
+ * doesn't need to do very much. It calls a single library function to ensure
+ * linkage works, but that is all. */
+int main()
+{
+    /* This version string is 18 bytes long, as advised by version.h. */
+    char version[18];
+
+    mbedtls_version_get_string_full(version);
+
+    mbedtls_printf("Built against %s\n", version);
+
+    return 0;
+}