Bump mbedTLS version to 2.28.0 (new LTS).

Keep applying the windows entropy patch (UWP support).
Remove no longer needed padlock patch.
Update thirdparty README to reflect changes, and new source inclusion
criteria.

(cherry picked from commit e375cbd094)

thirdparty/mbedtls/include/mbedtls/constant_time.h

@@ -0,0 +1,45 @@
+/**
+ *  Constant-time functions
+ *
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+#ifndef MBEDTLS_CONSTANT_TIME_H
+#define MBEDTLS_CONSTANT_TIME_H
+
+#include <stddef.h>
+
+
+/** Constant-time buffer comparison without branches.
+ *
+ * This is equivalent to the standard memcmp function, but is likely to be
+ * compiled to code using bitwise operation rather than a branch.
+ *
+ * This function can be used to write constant-time code by replacing branches
+ * with bit operations using masks.
+ *
+ * \param a     Pointer to the first buffer.
+ * \param b     Pointer to the second buffer.
+ * \param n     The number of bytes to compare in the buffer.
+ *
+ * \return      Zero if the content of the two buffer is the same,
+ *              otherwise non-zero.
+ */
+int mbedtls_ct_memcmp( const void *a,
+                       const void *b,
+                       size_t n );
+
+#endif /* MBEDTLS_CONSTANT_TIME_H */